What others have said:
if the West betrays and attacks the Soviets, then the civilians won't be behind the war, and the Soviets win. However, the West wasn't that stupid.
If the Soviets betray and attack the West, the West probably wins eventually, at great cost. However, Stalin wasn't that stupid.
To get this to 'work' you need some really tricky conditions. Maybe Stalin has a ?stroke? which massively increases his paranoia or something. How you get THAT neurologically, I don't know. Or, there's gradually increasing tensions and incidents near the end of WWII that leave both sides convinced the other guy betrayed them. That's possible, but really tough to pull off, as neither side wanted war, and both sides knew the incredible cost of continuing, so even provocations would be dealt with and smoothed over. Most likely.