Why would it be so hard to implement then? Just disable Dragon control for RTS, disable RTS control for player Dragon, send dragon player's input to RTS player's dragon actions, send RTS player's input to Dragon player's RTS actions, done! It's not that different from serializing enemy movements, wich already works. I think the real challenge would be to balance the gameplay.
If DC will contain this feature however, it might be the first time in history to mix RTS & 3rd person action in co-op (I never heard of it before), another title next to genre mixture.
But don't let your lead designer create any tullip soap bubbles if you believe you can't do it.
