Not exactly. Germany invaded on 1 September, USSR on 17 September, after Poland had more or less been defeated.
The Soviet rationale for their action was that they were acting to protect the (largely) Byelorissuan and Ukrainian population of eastern Poland against German aggression now that the Polish state had collapsed.
It was a pretty threadbare excuse, and if the Allies had been able to get there, they might have very well wound up at war with USSR when it invaded Finland.
Interesting that the Soviet Union didn't start fighting Imperial Japan until the US had nearly defeated her.
I sense a pattern here...
The Soviets invaded Poland because of an agreement between Molotov and Ribbentrop to divide the country. This was done so that Germany could invade Poland without opposition from the USSR; without that deal it is likely that the invasion would have been delayed or cancelled.
Well, they may have rationalized it in that way (I have no doubt they did), but the fact is that the Russians negotiated the partition of Poland BEFORE the Germans had even attacked it.
The Russians were nobody's savior.