The scene in Westminster has become a familiar Wednesday ritual: The man in the blue necktie, Prime Minister David Cameron, rises at noon to parry questions from MPs, while the man in the yellow necktie, his coalition partner Nick Clegg, sits quietly beside him, sometimes nodding in agreement and sometimes staring stony-faced.
But this week, the British Parliament's front bench instead displayed some of the fissures that are beginning to fracture the smooth face of the unlikely coalition that has united Mr. Cameron's Conservatives and Mr. Clegg's centre-left Liberal Democrats into a surprisingly resilient government.
On Wednesday, one Tory MP rose to ask a question about Britain's nuclear arsenal: Why hadn't Mr. Cameron fulfilled his promise of updating it? Had he caved in to the Liberal Democrats, who had long promised to abolish all nukes? The Prime Minister defended the arsenal and said its upgrade had simply been put off for budgetary reasons, but looked awkward, and a thin smile was visible on Mr. Clegg's face.
Then came the question neither of them wanted to hear. When Britain holds its referendum in May on switching to an alternative voting system, another Tory MP asked, would Mr. Cameron consider a strategy of throwing out the results if fewer than 40 per cent of voters cast a ballot?
The referendum – an idea hated by the Conservatives, whose party benefits from the old system – had been the Liberal Democrats' main condition for forming a coalition with the Tories eight months ago. In exchange, the centre-left party got a dozen cabinet positions and the deputy prime ministership, and agreed to go along with some Tory legislation that offended them.
The Prime Minister smiled wickedly and looked, for the first time that day, over to his yellow-tied companion. "We have not had thresholds in previous referendums," he cautioned the MP, "but the member should not be so down on this – I am sure he will work with me to get the turnout up for this one – particularly for the no vote!"
The Tories roared in approval. The Labour benches jeered: The coalition would soon be campaigning against itself! Mr. Clegg glared mutely.
Such moments have become increasingly familiar. The two leaders, almost identical in age, appearance, upbringing, education, social class and basic ideological outlook, have appeared at times this past year to be almost joined at the waist. Left-leaning Lib Dems and more conservative Tories have reacted with horror at the sincerely warmhearted relationship between them and at their frequent genuine agreement on policy.
But things have become less than congenial in recent weeks, and observers say they are likely to get touchier as the referendum looms over the alternative vote, in which voters rank each riding's candidates by preference rather than a simple X. (The system favours smaller parties such as the Liberal Democrats.)
Mr. Cameron and Mr. Clegg will be running nationwide referendum campaigns designed to make each other's parties look like dangerous threats or incompetent simpletons, while at the same time attempting to make a show of unity in the House of Commons.
"I think it's a far more robust coalition than many commentators thought it would be when it formed last year, and far more united in their views on core issues of liberalism," said the University of Hull's Matt Beech, the co-author of a book-length study of the coalition. "But there are certain policies that are going to be creating friction, that could damage the Lib Dems especially, and the alternative vote is one of them."
The other, an issue that could end up shattering the coalition, is tuition fees, which were divisive enough to erupt into violent riots in London before Christmas.
Mr. Clegg promised Lib Dem voters during last year's election that his party would abolish university tuition fees. This is not an utterly farfetched promise in Britain, where all universities were free until 1998, but it clashed both with the Tory plan of raising fees and with both parties' strong desire to begin cutting government spending and cutting deficits immediately.
Mr. Clegg remained silent as the tuition-increase bill was launched and protesters denounced him by name last year, but his party's cabinet ministers openly spoke out against the bill. This week, as Cambridge University announced fees of up to $14,400, there was the prospect of more protests.
There are other fundamental differences looming in the background. The Liberal Democrats are far more favourable toward immigration, and Mr. Clegg campaigned on a promise to grant immediate citizenship to all undocumented or illegal immigrants in a mass amnesty. Instead, he has gone along with his colleague's pledge to cut immigration.
While the two parties have managed to merge on a huge range of issues, the experience has not been good for the junior partner. The Liberal Democrats have been plummeting in the polls this year, largely as a result of their dramatic U-turn over tuition fees. Mr. Clegg desperately needs to show his party loyalists that he can deliver them something that is key to their beliefs, and the alternative-vote referendum is his main hope. If it fails, Mr. Clegg could become desperate.
He is already distancing himself from his Tory partners at every opportunity. "We will fight the next general election as separate and independent parties," he told the BBC last week. "Coalition is not a pact, it is not a merger, it doesn't mean that you become joined at the hip. It means that you retain your separate identities."