Reshuffles provide a rare opportunity for prime ministers to project their power. They should be moments to savour for the No 10 incumbent; they rarely turn out that way. Tony Blair botched most of his reshuffles; by the time Gordon Brown had his chance, his authority had collapsed.

David Cameron pledged not to tinker with his team every year. Mini-scandals have forced his hand, but he has kept rearrangements to a minimum, giving ministers time to delve into the detail of their briefs, making it harder for civil servants to dominate. Yet this first full reshuffle is more fraught than it needed to be. Two-and-a-half years into his tenure, Cameron has assumed the mantle of the great ditherer, presiding over a stagnating economy and bereft of ideas.

Cameron's choices go far beyond the fate of individuals – should Justine Greening stay at transport given her opposition to Heathrow expansion? Should Ken Clarke be put out to pasture? He needs to use the changes to provide new definition for the Government. But he faces a dilemma. Is his a right-wing, tax-cutting, welfare-slashing agenda? Or does he seek the mantle of the one-nation consensus politician with whom liberals can do business? Parliamentary arithmetic forces him in both directions.

The Coalition agreement provided a to-do list for the first half of this administration – sticking plaster to accommodate separate parties broadly united on deficit reduction. Now there is little to conjoin, with both leaders facing challenges from within their own ranks.

So how about this for an idea that will give both a sense of renewed purpose? Why not adopt an approach used by other coalitions, in which the junior party is given three or four ministries to run in their entirety? Give the Lib Dems the departments of, say, justice, foreign affairs, culture and climate change. By 2015 they could be judged against specific achievements (and failures), while the Conservatives could get on with their priorities without the frustration of having to look over their shoulders. For sure, there would be fierce battles in Cabinet – a mediation mechanism would be required that is more transparent and formal than the existing "quartet" – but at least something would get done.

Cameron's detractors put the indecision, the lack of measurable achievements, down to his lack of political hunger. He is too posh to push. His supporters blame it on the fact of coalition, the need to square big policies with Nick Clegg. Both propositions are likely to be true; but by removing some of the structural impediments, both leaders could show what they are made of.

Twitter: @johnkampfner john@jkampfner.net