Don’t worry about the Americans. If they split up their IPC’s between Japan and Germany, the Allies will end up loosing the fight. The Brits, if aggressive, can normaly only attack one of both Axispowers. Even if they sink your tranny and your sub, there’s nothing to be affraid of. You can destroy a lot with few efforts when the UK-fleet strikes at the Japanese in turn one, using your fighters.
The only problem is that if and when the UK strikes at the Japanese in UK1, you can probably expect a KJF. Then it’s up to the Germans to Panzer their way to Moscow.
If UK strikes the Japs on UK1, but then divert their offensive actions towards Germany, you shouldn’t have any trouble wiping them out of the Pacific and start nibbling on their income. Especially if USA splits its income between Germany and Japan.
Anyway, that’s what I’ve experienced in the games that I’ve played. An Allied player who tries to assault both Germany and Japans digs his own grave…