Why would Poland and Russia unit? Poland just fought of the Communists twenty years before and know that any 'alliance' will see Poland dominated by Russia. And Stalin would not be drawn into any war with Poland it can't win, Stalin is too clever for that, he knows that the USSR, in 1939, can not fight off Germany, especially in alliance with the west. Stalin's biggest fear was to be tricked into a war with Germany, that's why he delayed counterattacking during Barbarossa, he thought the Germans where trying to provoke a response so Germany could blame the USSR for the war. Stalin actually valued world opinion very much.
Also, Poland was under a nationalistic government itself, one that hated communists. It was a right-wing dictatorship, as i would presume it would still be. An alliance with communists didn't seem likely. The M-R treaty was different, both sides know what it was. It was basically a promise not to A.) For Russia not to attack Germany when it's engaged in the West B.) Germany not to attack immediately Russia (Stalin was ultra-paranoid about Germany allying with the West to invade the USSR) or help destabilize the Russian Government and C.) To setup a clear sphere of each countries future conquests in the East, since both sides know that Soviet action against the Baltic States was imminent.
If Poland did attack Germany, Russia wouldn't join, using the chaos to either try to seize the Baltics (if they can think they can get away with it) or just what for Poland to collapse and take Eastern Poland (like in 1939 in OTL).
People always assume that Stalin was some power-hungry madman without any sense of timing or any plans. This is a misconception. Stalin was power-hungry, but was he suffered from a classic state of Paranoia and was incredibly cleaver and calculating. His Paranoia made risky decisions nearly impossible for him to make. Stalin could do nothing without a well thought out plan. And he would have realized that taking on Germany with Germany having at least the West's moderate support, or worse, their direct involvement would end in disaster. That's why he didn't try to invade Germany in the 20s or the West Germany in the 40s and early 50s.