Were they mostly friendly, coolly accepting, or openly hostile? Did more ideologically motivated units react differently (ie SS divisions, or their Soviet equivalent [commissars?])?
Mostly on the coolly accepting side. For example, there was a joint Russian/German military parade in Brest Litovsk. Plus, there were examples of German forces ceding front line positions to the Soviets if they were fighting in what was to become the Soviet occupied zone. This was before the real explosion in size of the Waffen SS and so there were no SS divisions, though Liebstandarte troops did fight.
The most important thing to remember about the Russian intervention into Poland is how rushed it was. They had no clue the Germans were going to advance so fast and so far. In large part this is because they were distracted by the undeclared war in the east with Japan, which contributed to the delay in invasion as well as the rushed nature of it. The USSR and Japan only signed an agreement on September the 15th. They then declared the Polish government to be no longer in existence on the 16th and invaded the 17th.
Williamson, Poland Betrayed
Fairly isolated, but in the French documentary 'Apocalypse : The Second World War', there is footage of when the German forces and the Russians forces meet up, and they seem to have a good time socializing, sharing cigarettes and whatever else they had.
The Gestapo–NKVD conferences were a series of meetings organized in late 1939 and early 1940[1][2] whose purpose was to enable the German and Soviet security forces (the Gestapo and NKVD respectively) to share information regarding their operations in Poland. In spite of their differences on other issues, both Heinrich Himmler and Lavrentiy Beria had common purposes as far as the fate of Poland was concerned,[3] and the conferences discussed coordinating plans for occupation of the Polish nation and in fighting the Polish resistance movement,[4][5] which was an irritant to both Nazi and Soviet occupiers of Poland.
German–Soviet military parade in Brest-Litovsk (German: Deutsch-sowjetische Siegesparade in Brest-Litowsk, Russian: Совместный парад вермахта и РККА в Бресте) refers to an official ceremony held by the troops of Nazi Germany and the Soviet Union on September 22, 1939 during the invasion of Poland in the city of Brest-Litovsk (Polish: Brześć nad Bugiem or Brześć Litewski, then in the Second Polish Republic, now Brest in Belarus). It marked the withdrawal of German troops to the previously agreed demarcation line and the handover of the city and its fortress to the Soviet Red Army.