This is just so-o-o-o
fraught with issues...
What if Berlin falls in a fluke attack but is liberated at some point later? There'd come a point in the game where I'd almost want to leave berlin empty and dare them to take it just to eliminate the eastern front, get a suddenly unthreatened Josef on my side and let Japan concentrate on crushing the americans.
And the Western allies lose all units in soviet territory? So basically a precondition of any Western allied attack on berlin is that they have to vacate soviet territory or willingly lose those units?
And what happens if the Soviets
liberate berlin? Is it german? Are the soviets still axis? Are the germans now western allies?
And, and, and.....
The game is just not structured to deal with changing alliances.