Mary Ann Sieghart: Two of a kind - a coalition of style and substance

They are both public-school boys, tall, intelligent, comfortable in their skins, good-looking, and born just three months apart. They have highly competent working wives and small children. They would surely feel at home in each other's kitchens.

Yet Nick Clegg and David Cameron are not identikit products of the upper-middle-class, the public-school system or the British Establishment. And the subtle differences in their backgrounds explain a lot about the men they are today and the chances of their getting on with each other in government.

Both were brought up in the prosperous countryside within commuting distance of London: Mr Cameron in Berkshire and Mr Clegg in Buckinghamshire and Oxfordshire. Both were sent to prep schools that schooled bright, well-scrubbed little boys to excel in football, rugby and cricket and pass Common Entrance to Eton. The difference is that while Mr Cameron followed the traditional route, Mr Clegg instead found himself just yards from the Houses of Parliament at Westminster School.

He has described his prep-school education as "stiflingly conventional", while Westminster was "a breath of fresh air". Westminster has always been more iconoclastic and metropolitan than Eton, with a fair swathe of north London liberal intellectuals, and a teaching style that encourages boys to question and challenge. The Eton of the early 1980s, by contrast, was conservative and conformist. The only foreigners would have been sons of princes or sheikhs. Mr Clegg's cosmopolitan background would have made him a source of derision there – he would have been seen as an outsider, not fitting in to the rigid class system that Eton in those days cherished.

And while their parents were in similar jobs – both fathers in the City, and both mothers doing public service, Mrs Cameron as a magistrate, Mrs Clegg teaching disabled children – their outlooks on life were subtly different. The Camerons instilled in their son a paternalistic noblesse oblige, which insisted that privileged people had a duty to give something back to society. It was a philosophy of Anglican, one-nation Toryism. What Mr Clegg got from his mother, a low-Church Dutch progressive, was an unflashy but compassionate and open-hearted approach to life.

And the Cleggs, above all, were internationalists. How could they be otherwise, when she was Dutch and he half-Russian? They have a chateau in France and a ski chalet in Switzerland. Mr Clegg grew up bilingual in English and Dutch and later added French, German and Spanish. Such a cosmopolitan upbringing may seem commonplace now in 21st-century London, but in the home counties of the Sixties, Seventies and Eighties, it was downright exotic. Being a second-generation immigrant must have made Mr Clegg, in those days, feel like an outsider. Mr Cameron, by contrast, growing up in his Berkshire rectory, was the quintessential insider, as English and rooted as the roses in his parents' garden.

Mr Clegg's unconventional upbringing, particularly compared with Mr Cameron's, explains a lot about his future choices. While Mr Cameron trod the classic future politician's route of studying PPE at Oxford, Mr Clegg opted for the trendier but more esoteric social anthropology at Cambridge, where he read works by continental European intellectuals such as Claude Levi-Strauss. He then went on to an American university – again, rarer then than now – to write a master's thesis on the philosophy of the Deep Green movement which, in the late 1980s, was seen by many as the pursuit of cranks.

Both leaders were offered help by Establishment grandees to win jobs early in their careers. The Queen's equerry, a friend of Cameron's parents, phoned Conservative Party headquarters to recommend Mr Cameron. Lord Carrington, a friend of the Cleggs and a former Conservative foreign secretary, suggested to Lord Brittan, then the UK's trade commissioner at the EU, that the young Mr Clegg should join his private office.

Lord Brittan, who had been a Cabinet minister under Margaret Thatcher, tried to persuade Clegg to stand as a Conservative MEP. The two men, after all, shared a similar economic liberalism. But in those days, the Tories were socially illiberal and anti-European. That didn't fit with Mr Clegg's world view. So he joined the outsiders' party, the Liberal Democrats.

David Cameron's social liberalism has been a late conversion. He grew up as a county Tory, probably slightly uncomfortable with homosexuality and unexposed to other cultures. It was George Osborne, whose liberal metropolitan mother had been a great influence, who eventually persuaded Mr Cameron that the Tories had to catch up with the rest of 21st-century Britain. So while social liberalism ran in Mr Clegg's veins, Mr Cameron has had to adopt it with his head.

Mr Clegg has always been on the right of his party – as a contributor to the controversial Orange Book in 2004, he espoused a free-market approach to public services that was anathema to more statist Lib Dems. And Mr Cameron has, at least for the past few years, been on the left of his. So the two men are now converging, from different directions, on a shared agenda of economic and social liberalism.

So how will they get on with each other in office? Many policy differences remain, especially on Europe, where both men have unshakeably opposing views. But each will secretly be pleased that the coalition agreement has enabled them to ditch policies from the left (in Mr Clegg's case) and the right (in Mr Cameron's) with which they were anyway uncomfortable. The two men have more in common with each other ideologically than they do with people from the outer fringes of their own parties.

And what both share is an open-minded intelligence, somewhat unusual in politicians, which allows them to pore over a dilemma from all angles, to listen to other people's views and not to be stuck in a tramline of prejudice. Neither is too proud to ask for advice; indeed a self-deprecating charm is an attractive trait in them both.

Their London lifestyles are broadly similar: family houses in the leafy London villages of Putney and north Kensington. But in their constituencies, Mr Cameron reverts to his county Tory roots in the well-off shire seat of Witney, while Mr Clegg traipses up to the formerly alien environs of Sheffield. While this gives the Lib Dem leader an utterly different – and very useful – northern slant to British politics, it's not quite as gritty as it might seem. His constituency of Sheffield Hallam, packed with university dons, contains the highest percentage of upper-middle-class professionals in Britain.

Despite the different paths they have taken, you can easily imagine the two men enjoying a drink or having fun on holiday together. Although both can be slightly prickly if riled, they generally exude an easy charm that comes from social and intellectual confidence and a sunny outlook on life. Neither has adopted the arrogance that is so common at Eton and Westminster. Nor do they suffer from chippiness or insecurity.

What is more, you can tell a lot about a man by the woman he has married. Mr Clegg and Mr Cameron have opted for intelligent, independent women who are not afraid to challenge them. Both Miriam Gonzalez Durantez and Samantha Cameron have carried on working despite having young children and husbands with demanding careers. This is modern, 21st-century family life.

So could you imagine the two families tucking into bowls of pasta together round the kitchen table? Emphatically yes. While Mr Clegg and Mr Cameron diverged from their superficially similar origins to run different political parties, their lives have brought them back to the same place and, as TS Eliot might have said, to know that place for the first time.