i've done this a couple times :) actually, it's coming up on this saturday, my boys are turning 6(1/5) and 9(12/26)... i get nice paper and print out invites for classmates/individual friends with one child's name, do the same for the other child's classmates/individual friends, then i do a third set of printout invites with both kids names for family friends/church friends. hope that helps!
the only funny thing is i had a child rsvp via text last night that said "X will be attending your son's party on saturday" - after i looked at my invite list, i realized both sons have an "X" in their class! i did verify with the mom which "X" they were, and why i was asking, it was kinda funny :) all other rsvps have contained the bday child's name.