I haven't checked previous posts on this, but I thought I would put it forward.
This subject has been discussed many times before; the search engine, as the cliche has it, is your friend.
Obviously, Hitler had a frothing at the mouth hatred of communists and Slavs so it was probably never going to be a runner! However Stallin did not believe the pact was broken until the Nazis were well inside Soviet territory,he was paranoid about everything it seems except bizarely that!
Stalin was deeply suspicious of his allies and said as early as 1940 that there would be a war eventually; he simply got it into his head that the Nazis would never fight him while still at war with Britain.
Nazi-Soviet relations were not all hugs and kisses during the pact: the Soviets were inconsistent in delivering supplies, cheeky about Kybartai and Bukovina, and trusting enough to wait until the height of the French campaign for the annexation of the Baltics; the Nazis, for their part, stationed troops in Finland which set alarm bells ringing in Moscow.
I don't recall the opposition being as vociferous when the Stallinists invaded Finland and assisted in the invasion of Poland compared to the stern views taken of the Nazis!
Actually, the French and British general staffs both gave serious thought to the concept of a strike against Baku. You can see the sense of it from their point of view, but still, they might have paid more attention to the Nazi war machine in front of their noses. It must be remembered that a secret clause of the Polish guarantee explicitly made it a guarantee against Germany, and as for Finland, the threat of Entente intervention was one reason the Soviets made peace when they did.
Is this a scenario where the Nazis and Stallinists can prevail over all Europe?
Having the Soviets onside doesn't help the Nazis invade Great Britain - and I think they're bound to fall our sooner or later even if they do spend a spell as co-belligerants.
No doubt the Stallinists could be urged to become willing accomplices in the extermination of peoples who were targetted for extermination though the communists in this instance would be mostly theTrotskyists!
Actually, the Soviets handed over all German communists hanging around in their territory, including Jewish ones, but didn't turn back the large numbers of ordinary Jews who fled on foot during the partition of Poland.