If a model is in combat with a stunned or knocked down opponent and also with another opponent who is NOT stunned or knocked down, then he may NOT attack the stunned/knocked down opponent. When the other opponent enters, whose turn it is, or number of attacks does not matter, no attacks may be made against the knocked down/stunned model so no you can't split attacks to hit the downed model.
As for 2 knocked down models... that's an interesting question. If you have more than 1 attack can you take out 2 stunned/knocked down models in a single turn? I'd say yes, but you could probably argue against it.