Between who? Aliens are merciless, and have no known way of communicating with humans not to mention they have never shown pity to a human. Predators think of humans as say, deer. They go on a weekend trip to earth, kill a human so they can hang a head on the wall. The only way a Predator wouldn't kill you is if you were absolutly not worth killing. Like, say, and 80-year old blind man in a wheelchair is about as brag worthy as killing a deer who has no legs. Predators don't kill unworthy opponents 9 times out of 10.
So I would say there is a 0% chance of any alliance.
And if you are talking an alliance between Aliens and Predators, there is also no chance. First off, that gets rid of the movies point. Second off, the Predators consider them superior to all other races, making any alliance between anybody and them highly unlikely. third off, the Predators came to Earth for the sole purpose of fighting the Aliens.
