They were never on the same side. There were numerous treaties with Britain, France, etc that the Soviets proposed in order to take on the Nazis together, but they were denied, as western Europe and the US were doing a ton of business with Nazi Germany and were more opposed to the soviets. In fact, Western Europe already signed numerous non-aggression pacts with Nazi Germany, similar to the soviet-Nazi pact, but far earlier. The soviet pact was made on the eve of war to buy time, the soviets and Nazis never trusted each other.

This is only more clear if you look into how each country portrayed the other in the 30s. Communism and fascism are, again, polar opposites in property relations, they hated each other. The Nazis even saw Slavic peoples as being genetically inferior. Marxists were always opposed to fascism, and that didn't change when the Nazis came to power. They were never on the "same side," the soviets only went into Poland weeks after the Nazis did.

When it came time for World War II, all of that tension came to a head. The Nazis despised the communists above all else, and unlike Britain and France, tried to commit genocide against them. The communists despised the Nazis, and liberated Berlin years later.