Ups. Didn´t realise, that this had sparked such an intense debate.
Just to clarify: I was indeed thinking of two separate fronts to take out one player.
For example a player, that has a few territories in North America and a few in Australia (taking the old Classic map as example here). And I can deploy my new armies in a way, that will give me a 50 % chance on each side (chose 50 to make it easy).
And in hindsight the question seems rather dumb and the correct answer (25 %) rather obvious to me.
Cheers.