Japan SHOULD be able to go to war against the Soviet Union. That’s the nature of the game. It was also a serious option Japan considered for quite some time. Indeed, in the end they decided to go for the Southern option. It’s difficult to simulate both things in a game of the nature of A&A. One we want mutual attacks to be possible, Two neither of the two countries in question was eager to fight the other while already at war with Germany/Italy or the US/UK/France respectively…
Perhaps a negative NO would work or an IPC penalty (in the rulebook it’s explained that NOs represent a boost in morale and vigour (I suppose both in the military and as far as civilians/factory workers etc. are concerned). Hearing that your country is being attacked from both sides would probably have the opposite effect of an NO, so a negative one might work: -6 IPC if Japan is at war with the Western Allies and the USSR simultaneously and -6 if the USSR is at war with both Japan and Germany/ Italy. These negative SA’s only apply when you initiated the conflict with the Soviet Union or japan respectively. ( I agree the wording needs to be worked on, but I’m sure you get the idea). I’m certainly not for completely disallowing the possibility!