I don't see how they could attack seperately, since Poland is part of the alliance (if it has any chance) and therefore Germany + SU will attack. Perhaps a strong baltic could stop the M-R pact, or maybe a different one?
Bring them all to the Allies (feasible since nobody would be at war with Germany yet) or take Poland out via an event, not really that difficult.
Not sure if Poland would be an allies member, though, if Germany, again, hasn't attacked any nation yet.