Some examples:
Crossover: Ranma and Akane go camping with Tenchi
Masaki. (Tenchi Muyo crossover)
Fusion: Agents Ranma and Akane of the JIB fight
alien criminals loose in Japan. (MIB Fusion)
Crossover: Akane meets Belldandy while shopping,
and gets a wish. (AMG/OMG crossover)
Fusion: Ranma, God of Whatever, is directed by
the Goddess Relief Office (under protest) to grant Akane Tendou a wish.
(AMG/OMG Fusion)
And thats all there is to it. : )