Actually the Soviets already defeated the Germans. 80% of German forces were on the Eastern front and Russia was cutting through them like a hot knife through butter before d-day even occurred. They took Berlin despite the German high command basically trying to give it to the Western allies. The turning point of WW2 was Stalingrad, not D-day.

it is interesting to note that at no point during the war both sides fought simultaneously against the other side. E.g., the USSR was on Germany's side during the battle of Britain (supplying fuel for german bombers). The Japan non-aggression pact was kept until after the defeat of Germany. The many European countries that fought on both sides along the course of the war, etc.