I can imagine two scenarios;
1. They get a common enemy. Britain or Russia is a good bet, or for extra effect you can even have Britain AND Russia against the Franco-German alliance.
2. Germany wins WWI, annexes Briey-Longwy. Why is this important, you ask? Because most of the French heavy industry is there; without it, they don`t have the means to start a revanchist war. France becomes a former great power with a gigantic colonial empire, and eventually gets to join the economic union the Kaiserreich sets up, most likely unwillingly all things considered.
For added effect you could make Russia go Red (again) and make it look like a threatening power; that might make the Westren Europeans fear the Red Menace more then they dislike each other, thus forcing them into an alliance.
As for how the alliance will work, I think the initial distrust and outright hostility between the two would go down given time spent in an alliance, and especially if they fight a war against someone together.