MrCleveland
This only happened in Roger Rabbit but...
Disney vs. Warner Bros.
I can see Bugs and Mickey laugh at Donald and Daffy fighting with each other, Sylvester teaching Figaro how to chase mice and birds, Tweety and Timothy would outsmart those two, The Disney Pricesses getting advice from Lola Bunny, Elmer Fudd trying to hunt Bambi, Speedy Gonzales and Panchito have a race on what's faster, Chip and Dale team up with the Goofy Gophers, the thoughts would be endless!
Disney recently had a crossover with Square Enix which is a Japanese RPG.
I was referring to as Kingdom Hearts (for PlayStation 2) which expanded into a franchise. Kingdom Hearts had Disney characters (Donald Duck, Dumbo, Bambi, Simba, Mushu, etc) forming a alliance with the Final Fantasy video game series. New characters were written to the game (Sora, Kairi and Riku).