Why? If their objective is taking Eastern Europe (and maybe the Balkans), why would they suddenly decide to subject France and Britain? If the Western Powers decide to attack the SU because of its expansion, what`s stopping the Soviets from laying back and waiting for the Western Powers to come to them?
I was assuming Germany would get itself involved. In case it doesn't, then by all means, the Soviets would likely just stop and try to get the allies to accept the new status quo.
How successful the attempt would be remains a question.
This is assuming Germany is a direct goal of the Soviet advance, in which case your scenario would happen. But if the Soviets dont plan on marching below the Branderburg gate, then why would the Wallies and Soviets have a deathmatch in Thuringia?
Because once the Soviets go on the offensive Germany will panic?
Then again, it depends on how far the USSR pushes before coming down on Poland. If it deals with, say, Japan/Manchuria, then Finland, then the Baltic states, all with the stated goal of exporting communism chances are Germany will get involved as they're going to see Poland as merely a stepping stone on the way for the bear with Germany as the next stop (which, mind, it could very well be). Besides, it's better to fight a war on someone else's turf and use the time to build your own army up to scratch.
If it's Poland first and the Soviets offer Germany ... say, 1914 borders, I'd wager Germany might (depending on the government in power at the time) jump on the offer and then you'd have the funny war where neither side can fight the other.