I know this is a kinda old thread, but incase others are wondering the same thing I might have a soloution. You could always try it like it would be in the real world. Like take(using the original names for lack of original ideas for that at the moment) Phillip Fry and Amy Wong and "marry" them. So then you'd have Phillip Fry and Amy Fry. Then they have a kid who gets the same last name and for each marriage the last name of the female gets changed to the last name of the male. That would probably be the best bet in my opinion.