AHC: Keep World War II as two separate wars

[Zaius]

Really, the hardest part of that thing isn't the USSR jumping in. It's Poland siding with Germany: it could happen, but you would have to have the Poles get a somewhat more belligerent streak then IOTL and really aggravate the Soviets and the Anglo-French with the annexation of that slice of Czechoslovakia they took IOTL.

It is indeed hard to make Poland side with Germany in this scenario, but it is also not necessary. The OP wants the USSR to take part in the war, but does not specify how big the Soviet contribution should be. Stalin responding to the French declaration of war by declaring war himself, and then lurking behind the barrier formed by neutral Poland throughout the war, conserving his strength while proclaiming his full support for the Allied cause, maybe sending over a trickle of support through Romania which was apparently ready to tolerate this in OTL, fulfils the OP's conditions. And coincidentally vindicates Poland's "equal distances [between Berlin and Moscow]" doctrine. (If Czechoslovak forces are greatly weakened and pushed into Slovakia while a phony war rages in the west, let's say until the summer of 1939, can the trickle of Soviet troops ultimately cause Czechoslovakia to go red?)

A possibility, although I would observe that the Soviet-Polish War technically started with a Polish invasion of the USSR.

Fighting between Poles and the Red Army started at the very beginning of 1919 when the Red Army seized Wilno. Between the 1919 and 1920 campaigns there was a de facto armstice broken only by sporadic fighting and negotiations which ultimately went nowhere, but no peace treaty. So who started it is at least debatable.

Well, my point is that the Poles may align with Germany vs. the USSR (OTL they aligned with Germany to get that little slice of Czechoslovakia after all...) and it's not beyond realm of possibility.

The Poles were very much against limiting their soverignity if it seemed avoidable. To get them to actually align with Germany requires German blackmail. And if Germany is already fighting a war against multiple powers Hitler will have other things to worry about.