OK, so we've got the best of both worlds sort of dual classing going.
Best HP, best proficiencies, best skills (acquire bonus skills from both classes, and spend training slots freely between the two?), best number of Healing Surges, all class features.
You get to choose an Encounter and Daily from each. But do you get to use both? Or could you just choose one on the fly?
At-Wills. Only two, but any combo from each class? Gain a bonus At-Will? Or have access to 4 At-Wills?
I'm inclined to leans toward only getting to use one Encounter and Daily, but getting to choose which one on the fly. And only two At-Wills, but can be from either class.
Aside from that, multiply HP by 5, gain a +5 to saves and gain one action point each encounter instead of every other. Be limited to one action point a round, but any number per encounter?
Should build one hell of a resilient character. Not sure how'd they fair against encounters built for a party of 5, but could be fun to test.
EDIT: Initial Thought - Pally/Cleric would be mean and well synergized. Best armor, HP and Surges. Lots of self healing potential. Good access to blasty type powers and melee. Divine Challenge becomes pointless, but it's a small sacrifice. Maybe class train to Warlock for more CHA goodness.
Additionally, the classic Fighter/Wizard. Scale, best HP, increased Surges. Good single target melee and AoE for the larger fights. Class train Warlord for tactical maneuvers to move your enemies and Inspiring Word healing.