My play group always splits the Allies into the two respective theaters in the same way that the Axis are split. The Pacific player has control of everything that happens on that board, likewise for the Europe player.
Of course, this means that two players end up playing the USA simultaneously. Some discussion goes back and forth over how much funding each side of the board should receive given the state of affairs, and usually one side or the other is focused on. I like this a lot because it forces the two Allied players to really put their heads together over what kind of game to play, each bringing their evaluation of their side of the board to the table. Also, some jokes get flung around about requisitioning funds, failing to put the paperwork in, congressional debates, etc. etc.
Usually Russia and the U.K. stick to the division of force that’s already in place, but sometimes U.K.'s Pacific forces get called over to Africa, or perhaps Moscow funnels some IPC’s to support the far eastern troops (or the troops march home).
In keeping with this tradition, one game the Japanese player flew 4 bombers over to help the Germans and then relinquished control of the units to the German player (to be moved during Japan’s turn, of course).