Only one person is ever in charge at 10 Downing Street - but what is decided behind the famous black door is now the responsibility of two men, and two parties.
The two men in question, David Cameron and Nick Clegg, barely knew each other 100 days ago.
Their two parties, the Conservatives and the Liberal Democrats, knew each other only too well.
Apart from a few isolated examples in local government, they had been at each other's throats for years.
So, as this most unlikely of political alliances reaches its symbolic 100-day milestone, should Mr Cameron and Mr Clegg be celebrating the successful birth of a new kind of politics, symbolised by their joint press conference in the Rose Garden at Downing Street on day one?
Or should they be letting out an exhausted sigh of relief that they made it this far?
The chaos and calamity some, including the Conservative Party in a general election broadcast, warned would result from a hung Parliament has not yet come to pass.
But there are still formidable hurdles to clear before this experiment in coalition government - the first at Westminster since World War II - can be judged to have been a success.
'Further and faster'
Top of the coalition's list from day one was getting to grips with the deficit, the gap between what the Treasury takes in tax revenue and how much taxpayers' cash the government spends.
At the general election, the Liberal Democrats continually questioned the Conservatives' commitment to go "further and faster" than Labour was already planning.
But in the days after the election, it became clear to political insiders that the Lib Dem leadership was shifting its position.
Senior Conservative John Redwood, who was concerned about working with the Lib Dems, credits their change of heart with allowing the coalition to get off the ground in the first place.
He told the BBC: "I argued the case immediately after the election that we should try for Conservative minority government, with the Liberal Democrats allowing us support in motions of confidence and to get through the main parts of the budget, because I didn't think at that point that the Liberal Democrats would buy into the kind of Budget we were recommending.
"When I discovered that the Liberal Democrats were coming around to the view that we needed a tougher and tighter Budget, then it was clear you could go further than just having a minority government."
'Peas in a pod'
So, even from day one, in the light-filled Downing Street garden, the message was less sunny.
Like Labour, the coalition believed major reductions in public spending would be necessary to deal with the public finances.
But the pace and scope of cuts would be faster and deeper - the coalition's ambition is to eradicate the deficit completely by the end of this parliament. Coalition ministers presented a united front on the need to act now, and act boldly.
Indeed, when it comes to those seated around the cabinet table or in junior government roles, the two parties have gelled surprisingly well.
One cabinet minister, describing the body language between Clegg and Cameron, told me they were like "peas in a pod".
It probably helps that there are 23 Lib Dem ministers - including nearly half of the parliamentary party - scattered around various government departments, with the deliberate intention of embedding the party in the administration.
But for all that many of the Lib Dems and Tories with government jobs appear to be thriving, in recent weeks there has been a tacit admission that they have not done as well as they might have in selling their core message to the public, many of whom are nervous about the impact of the squeeze on spending.
Lib Dem anxiety
The speed at which Chancellor George Osborne wants to balance the nation's books is just one source of nerves for Lib Dem MPs - particularly among those who remain on the back benches, without a government position.
There has been anxiety over the Academies Bill, a flagship Conservative policy, that will reform education in England.
There has been nervousness about the scale of the reforms to the NHS in England - a plan that was not in the coalition deal thrashed out in the aftermath of the election.
It should be noted some of the other tensions in government are traditional disagreements between departments, not coalition clashes - differences between the Treasury and the MoD over who pays for Trident for example.
But, equally, it is true to say that anxiety about the coalition runs deeper among Liberal Democrats than it does among Conservatives.
Many harbour a fear about losing their identity; being swallowed up by the party that outnumbers them at a rate of more than five to one in the Commons.
Their dip in the opinion polls does not do much to bolster Lib Dem confidence either.
Former leader Sir Menzies Campbell has warned Nick Clegg about entering into any kind of electoral pact or deal with the Conservatives, as some have suggested could be the answer and which would see the Lib Dem candidates not fielding candidates against Tories in areas where they could not win, and vice versa.
Sir Menzies said it should be a matter for local parties, not the leadership, to decide: "He might issue an exhortation but he could not issue an instruction.
"Every Liberal Democrat leader will tell you that [backbenchers] find the carrot a much more attractive instrument than the stick."
As the two parties move from their first 100 days shackled together in coalition towards the party conference season, there will be an inevitable focus on relationships inside the parties.
Both David Cameron and Nick Clegg are understood to be having to spend more time than ever before cajoling and soothing egos, keeping their MPs and activists on board, using the "carrot" to the best of their ability.
But that internal discomfort could well be dwarfed by the public's reaction when the cuts announced in the Budget and October's spending review begin to bite.