In 1658 King Frederik III of Denmark signed the Treaty of Roskilde, handing over the city of Malmö to his nemesis, King Karl X Gustav of Sweden. But, it seems, no one has told the people of Malmö and Copenhagen. Today, many think of Sweden’s third-largest city as a suburb of the Danish capital.

Malmö’s nickname, “København M”, says it all: the two cities now form a single conurbation, combining the low cost of living and relaxed atmosphere of the Swedish city with the higher salaries and employment opportunities of Denmark’s capital. “The citizens of the two cities shouldn’t feel there is any border,” says Ilmar Reepalu, Malmö’s mayor, who meets monocle in Copenhagen’s town hall, where he is putting the final touches to a joint 20-year plan for the two cities: “We are aiming for it to become a single metropolitan area.”

It is the first time Copenhagen and Malmö have integrated their futures to such an extent but the germ of their co-operation dates back almost 20 years. “In the mid-1990s, Malmö’s [shipbuilding] industry collapsed,” says the former architect who took office in 1994. “But we created a joint port with Copenhagen, with 50-50 Danish-Swedish ownership, and it’s been a great success.”

It is easy to see why Reepalu has developed alliances with the Danish capital. Geographically, his own seat of government is nearly 650km north of his home in Skåne County. In contrast, Malmö and Copenhagen are visible to each other across the Øresund Strait – the stretch of water that separates the southern tip of Sweden from the southwestern shores of the Danish island of Zealand.

A large part of this dynamic cross-fertilisation is down to hardware and the region’s engineering feats – the 16km Øresund bridge opened in 2000, providing a fixed road and rail connection between the two settlements. There are now over 9,000 Danes living in Malmö (a threefold increase since the bridge opened), over 20,000 commuters cross the bridge every day and Danish is taught in some schools in the city. The inhabitants of the two cities can even borrow library books from each other.

The alliance is so successful that many have questioned national loyalties. “Malmö is not becoming Danish,” Reepalu tells monocle. “I don’t think the state plays an important role in regional identities. When Stockholm feels the need to say, ‘We are the capital of Scandinavia’, I think that shows a sign of weakness.”

Regardless of any historic enmities the two countries might have had, the mayor feels the two cities are better off as one. He admits that as the junior partner, Malmö would be expected to gain the most but points out that as the fastest-growing city in Sweden, it offers Copenhagen access to a larger workforce and knowledge market with which to attract foreign investment.

Though it has had well publicised problems with crime and social unrest in the past decade, Malmö is buzzing with large immigrant and student populations. Since the redevelopment of its Western Harbour – with Santiago Calatrava’s Turning Torso building as its centrepiece – it has been seen less as Copenhagen’s little brother and more as an equal partner.

Another area where the twin cities are thriving is education. The two boast eight universities between them and the region is one of the top five research areas in Europe. Mayor Reepalu is pushing for further integration of the education systems for older age groups. “We have 165,000 students at university level in the region, and today 2,000 students are going to university on the other side,” he tells monocle. “That’s only 1 per cent, so you could say we haven’t managed so much. The Danish government recently passed a bill that makes it even more expensive, so we have to work on reducing the cost and other barriers.”

One student who does make the journey by train every day from her home in Malmö is Ebba Tham, who attends the Royal Danish School of Architecture. “The school is very different from the one we have in Sweden. It’s under the academy of arts and the one in Sweden is under the faculty of engineering,” says the architecture student. “It’s more artistic and philosophical and by being in Copenhagen, a capital, I think the school is more engaged in the city.”

One Copenhagen town hall insider tells monocle that they like to think of the two sides of the Øresund in the same way as San Franciscans think of the two sides of their Bay. “Malmö is growing and shaping its own future and I love that feeling of development,” says Daniel Zander, another Swede who travels across the Øresund to his job as a senior art director at the digital agency, Creuna. “The mix of people and cultures here is something you’ll not find anywhere else in Sweden. But Copenhagen is a European capital; my kind of job would be hard to find in Malmö.”

Thousands of Danes have moved to Malmö, tempted by cheaper rents and the easy 33-minute commute. Klaus Sindahl, a Dane who works as a mergers and acquisitions manager for Novozymes in Copenhagen, says, “Family life is easier in Malmö. We bought a house, which we would never have been able to afford in Copenhagen. A lot of large companies are moving to Malmö – ikea moved one of its hqs here. It’s getting more interesting all the time.”

In many ways, the bilateral urban alliance is a triumph of regionalism over national boundaries. Malmö and Copenhagen lie at the heart of the ‘Øresund Region’, a joint initiative to brand the new twin city area. “The idea of the “Øresund Region” was to reap the full benefits of the new infrastructure,” says Finn Lauritzen, new director of the Øresund Komiteen, which oversees the co-operation. “We had the bridge, but also this huge benefit that Danes and Swedes understand each other’s languages. There are few areas in the world with this advantage.”

Like Mayor Reepalu, Lauritzen is ambitious for further co-operation. Both would like to see a joint tourist board and a metro link underneath the Øresund within the next 10 years. His vision is for a single conurbation to compete with major capitals like London and Berlin. “I want the barriers to come down in terms of a single labour market,” he says. “At the moment it is complicated to live in one city and work in the other. In the long term, I want to create a clean tech cluster that is the strongest in the world.”

In many ways the two cities’ contrasting cultures are what make the trans-national bond so successful. Dan Storbæk, Danish boss of Malmö-based tech start-up Skarpline, points out that while the Swedes have 16 of the largest 25 firms in the Nordic region, the Danes are better at small-to-medium scale businesses. “People in Denmark are expected to be more loose, informal and to the point, as well as more sales-oriented,” he says. “In Sweden, they are seen as more diplomatic and consensus-oriented.”

The traffic between the two cities is not entirely one-way. Some 700 commuters live in Copenhagen and work in Malmö. “Often Swedish companies like having a thicker-skinned Dane at the top,” says one Copenhagen-Malmö commuter, Thomas Marschall, Danish ceo of Swedish company Precise Biometrics. “Danes often bargain and negotiate harder than Swedes. The stereotypical Danish ceo in Sweden has a reputation for being meaner and less sensitive to hiring and firing. I wouldn’t say I was chosen just because of my nationality, but after I started, I think the shareholders appreciated my, let’s say, ‘brisk’ Danish approach.”

Copenhagen-Malmö commuter, Kristina Gordon works as a visual designer for the Malmö branch of UK design firm, ustwo. “The paperwork with tax and so on is still very difficult but I really like working here,” she says. “Malmö is cosy but there are really good shops. The Swedes take a proper one hour lunch break and go out to eat, whereas in Copenhagen it’s half an hour sitting at your desk.”

The political will in both cities is leaning towards much greater integration – there is talk of closer co-operation between the healthcare systems of both cities and Copenhagen mayor, Frank Jensen has written extensively about his vision for the Copenhagen-Malmö partnership to be the growth driver of the region. “I am convinced that Copenhagen benefits from the close co-operation,” Jensen tells monocle. “The Øresund Bridge alone has given us new jobs and economic growth worth dkk30bn (€4bn). We want even more integration, so we are co-operating with our Swedish colleagues to reduce cross-border obstacles.”

It is clear the urban pairing is in both cities’ interests. Copenhagen, one of Europe’s smallest capitals, can use its relationship with Malmö to bolster its weight and compete on the international scene. At the same time edgy Malmö can reap the benefits of its proximity to a world-class capital.

A highly competitive Danish-Swedish metropolitan region looks set to emerge, with joint ports and synchronised transport networks, and with education, healthcare and tourist services set to follow. Like all of the best relationships, the locals of both cities cannot fail to benefit.

Vienna/Bratislava 

The Austrian capital and its Slovakian counterpart are rediscovering a beautiful relationship.

Like real twins, some twin cities get separated. Vienna and Bratislava are naturally divided by about 60km, a language, and a national border, but also by history and ideology: Bratislava lay behind the Iron Curtain for nearly 50 years when it was Czechoslovakia’s second-biggest city. But before that, there had been a clear link when both cities were neighbours in the Austro-Hungarian empire.

In recent years, there’s been a reunion between the two cities, both of which are on the Danube River. Reconnection began when Slovakia became independent in 1993. Slovakia joined the EU in 2004 and adopted the euro in 2009, enabling economic exchange and investment opportunities. A highway between the cities opened in 2007. With firms such as Volkswagen and Kia now established here, Bratislava has become a car industry hub, while Vienna and its environs are booming in biotech and research. Multinationals such as Samsung and Hewlett Packard see the advantage of the greater “Twin Cities” region, but even smaller firms like the Viennese hatmaker Mühlbauer cross boundaries, with some of the company’s hats now being made by Bratislavan tailors.

The two cities’ chambers of commerce, industrial organisations and tourism boards now work together and the region has the potential to become a super-efficient transport axis, with synergies between ports, better use of airports and new rail projects in the works. But perhaps the region’s best resource is its educated inhabitants – the joint student population is about 250,000, some of whom live in one city and study in the other.

Brazzaville/Kinshasa 

The two African cities don’t have much in common, apart from a history of civil war but moves are afoot to bring them closer together.

The world’s closest capitals, Brazzaville and Kinshasa, regard each other with some suspicion across the wide, brown expanse of the Congo River. Separated by 3km of water, the two cities have developed very differently. Brazzaville, capital of the Republic of Congo, has maintained many colonial trappings, having the atmosphere of a provincial French town dozing in the tropical heat. Kinshasa on the other hand is the chaotic home to 10 million souls and cultural heart of the Democratic Republic of Congo, a seething, febrile place famous for its music, nightlife and sporadic violence.

Both Congos have known civil war, with traders taking advantage of conflict-induced shortages on either side of the river, opportunistically shuttling goods in the rusting, overcrowded ferries plying the crossing. On occasion in the 1990s, the two countries even traded fire, launching rockets at each other across the water.

Relations have improved, but the crossing is still not for the fainthearted. Safety concerns aside, the “beaches” in each city, where boats dock, are a whirl of semi-legal officials demanding taxes, payments and bribes before you stumble out in the sweltering heat, lighter of wallet but clutching your documents. There is talk of building a road and rail bridge to connect the two cities but these kinds of projects usually take years to realise, if they happen at all.

For now, the crossing is an exhilarating experience but also a stark reminder of the difficulties Africans face in moving people and goods across borders.