Inspired by current discussion over the "Poland attacks first" scenario.
Your scenario: in August-September 1939, Nazi Germany and the Soviet Union make it clear beyond any reasonable doubt that they are together in the solution of the Polish crisis according to their interests.
They syncronize and mutually support their claims against Poland, the public part of the Molotov-Ribbentrop Pact may include an anti-Polish "defensive" alliance or at least a joint statement of reciprocal support.
If the war occurs, they invade Poland from both sides since the very onset.
Does Poland submit to a new partition ? Do they fight a hopeless war ? Do Western Democracies acquiesce or fight both totalitarian giants ? How and when the unholy alliance breaks down ?