I'd add the class skills, hit points, and powers from both classes. Sure, why not? 4 PCs with X hit points and Y powers should be roughly as effective as 2 PCs with 2X hit points and 2Y powers. I'd give them extra action points too: let's say, they start the day with 2 points and gain 1 per encounter, and there's no limit to how many they can use in a single encounter. Also, give them +2 to all saving throws.
Viola! Your PCs are now Elite creatures. Count them double when calculating encounter XP budgets (so 3 Elite PCs would be facing an encounter suitable for 6 normal PCs), but they get half XP (so 3 Elite PCs would each get 1/6 the XP rather than 1/3).