Two of the most recent examples that I can think of is Lopez vs. Lopez starring George Lopez and his real life daughter Mayan and Poppa's House starring Damon Wayans Sr. and Damon Wayans Jr. There's also Unstable starring Rob Lowe and his real life son John Owen Lowe and Hannah Montana starring Billy Ray Cyrus and his real life daughter Miley Cyrus.

Going back further, there's Diagnosis Murder with Dick Van Dyke and his real life son Barry Van Dyke, In the Heat of the Night with Carroll O'Connor and his real life son Hugh O'Connor, Here's Lucy with Lucille Ball and her real life children, Desi Arnaz Jr. and Lucie Arnaz, and the entire Nelson family on The Adventures of Ozzie and Harriet.

If you want to count actors and their real life stepchildren, then there's Shirley Jones and her real life stepson David Cassidy on The Partridge Family.

This is a hard question since the criteria is staring together not one guesting on the other's show.

There are a bunch of choices for one guesting on the other's show. Like Danny and Marlo Thomas did this 3 times. Once on the pilot to The Joey Bishop Show, once on The Practice (the 70's comedy), and twice on That Girl.

I'll have to think about this. I can't imagine a large number of offerings here.

Still the Beaver/The New Leave It to Beaver - Ken Osmond (https://www.metv.com/stories/apparently-ken-osmond-was-as-far-from-eddie-haskell)'s real life sons (https://www.youtube.com/watch?v=CEqmXz0Quks) Eric and Christian (https://www.nickiswift.com/210526/the-untold-truth-of-ken-osmonds-sons/) also played his sons in the Leave It to Beaver revival series (https://www.youtube.com/watch?v=eAHm-WSCWF4) from the 1980s.