Effectively you need a specific amount of VC’s in either theater for the Axis to win with the normal rule set.
As such, Japan can end the game all by itself and Germany can just turtle or do whatever it takes to distract the US from spending IPC in the Pacific (IE SeaLion that ends up off Gibraltar in position to lay siege on US territories).
So, if you change it so that the Axis win is a combination of VC’s in BOTH theaters, you force Japan to play its Pacific Agenda while Germany goes after its European Agenda and you don’t have a Japanese fleet in the Med on Round 8.
It also prevents Japan from giving up easy to hold VC’s like the Philippines or the ones in China and instead going all-in against Moscow on a long march or by putting an AB somewhere in China and flying 15 aircraft to Stalingrad the same round Germany takes it.
So if Japan needs “x” VC to win the Pacific and Germany needs “y” VC to win in Europe, you can come up with some combination of X and Y so that when the Axis arrive at something like 15 total VC, the Axis wins the game, regardless of which VC the Axis controls.
Of course, that opens a whole new basket of eggs for strategy where the Axis may be able to win the game without taking out a single capital from the Allies. But you could always require that at least one Allied Capital is lost in each theater to prevent “cheap” wins.