Remember, acting together in an episode...

Billy Tate's Jimmy Baio and 80s teen hearthrob Scott Baio appearing in an episode of Fantasty Island...

http://www.youtube.com/watch?v=EeBKohb6DDU


The Oslen Twins on Full House

Lucille Ball and Desi Arnaz Jr. on I Love Lucy, though I don't know how much just standing there counts as acting (he was only 4 at the time). So if not, he later appeared on The Lucy Show in the episode "Lucy is a Referee."

If husbands and wives count, then Lucy and Desi Sr. were first together in the 1940 film Too Many Girls.

Tia and Tamera Mowry on sister, sister
umm and Kirk Cameron guest starred on his sister Candace Cameron's show full house