The German military was not capable of fighting a two front war at that time. Frankly, had they known the French and British would actually declare war on them for going into Poland they probably would not have. They thought it was a bluff. If they were caught off guard by war being declared, imagine their shock if that declaration had been swiftly followed by France, mobilizing as fast as possible, mimicking a blitzkrieg to Berlin, supported by whatever the British could add to the majority French effort.
Yes, in reality there are multiple things that blocked such an action being possible, but that is not the purpose of running "what if" mental games.