Mediocre, ultimately disappointing idea? Or GREATEST IDEA EVER!? It'll prob be one or the other.
They cross paths with Homer, who graciously greets his new “albino” visitors, and the two families become fast friends. Stewie is impressed with Bart and his assortment of pranks, Lisa befriends Meg and tries to figure out exactly what it is that she’s good at, and Marge and Lois put aside their housework in favor of some bonding. Meanwhile, Peter and Homer argue over which beer is better: Pawtucket or Duff.
Sounds promising. The Simpsons had one crossover episode in the mid 90's that featured Jon Lovitz guest starring as his character Jay Sherman from The Critic. Family Guy hasn't had any, though they did have that excellent South Park Episode about their writing process.