Yes, but the problem is that in doing so they miscalculated. Now that all the polls show that Clegg came out of the debate the hands-down winner, Labour and the Conservatives know that by focusing solely on each other they allowed Clegg to appear reasoned, impartial and above the 'petty feuding' of party politics. Both Brown and Cameron want to win over swing voters, and they can't do that if they just attack each other and allow the undecideds to fall into the arms of bipartisan 'not Cameron or Brown' Nick Clegg.
I'm not saying that changing tack will work - it may be inevitable that they become sucked into the same sort of 'one on one' mindset next time. However, I guarantee you they will change tack. These guys are extremely well-prepared, and the people running their campaigns aren't stupid - they know they can't just pretend Clegg isn't there next time, because if they do, the result will be the same.