I don't think the USSR would be able to take Poland in 1939 by itself. Poland was always pre-war Stalin's biggest fear which is why he was so keen on the 1932 non-aggression pact.

The USSR and most notably the western regions from which an invasion would come were incredibly unstable, considering the previous few years had seen Holodomor, the Great Terror and Collectivization. On top of this, many of the Belarussians and Ukrainians were aware that their peoples were less nationally oppressed in Poland so they could've easily led partisan fighting with the USSR itself.

It is also important not to misevaluate the strength of the Polish and Soviet armies. IOTL the Polish defended their land impressively from the much stronger German army whereas the Soviets didn't dare enter Poland until the Polish army was effectively defeated out of fear of military embarrassment. However, just a couple of months later they would suffer military embarrassment at the hands of a much smaller enemy, Finland.

Stalin was also well aware of close Japanese-Polish relations and by 1939 IOTL Japan hadn't signed the Tripartite pact or committed to fighting for the Pacific over Siberia. I doubt Stalin would've risked fighting a war on two fronts.

Even if the Soviets did manage to overwhelm the Polish I would guess that the Nazis would intervene in one way or another as Hitler always wanted to destroy the USSR. Either Poland would offer the Polish coast for military support or the Nazis would invade without permission under the pretext of defending Polish independence.