Last month my wife and I visited a newly opened WWII museum in Warsaw, Poland. It depicted the Warsaw uprising which occurred rather late in the war [not the Jewish uprising which occurred earlier].
Our guide described the events leading up to the invasion of Poland simultaneously by both the Germans and the Russians.
The Polish student that was with us told us that his grandparents had fled from the invading Germans by taking a train to the east. They arrived on the Russian border on the same day that the Russians were invading by train. His grandparents hopped on the first train back to Warsaw. His grandfather was captured by the Germans and forced to work on a German farm until the end of the war. He said that his Grandfather was treated well by the farmer and released just prior to the allies invasion at the end of the war.