What show featured 2 guest stars that were main stars together of their own show in which the star of the show in question played a minor recurring character in the 2 guest stars' show?
I can only think of one show.
I have attempted, unsuccessfully so far, to prove you wrong that this is the only such show. I thought that
Friends would meet the conditions, as Lisa Kudrow played a recurring minor character on
Mad About You, and Helen Hunt and Leila Kenzle were guest stars on an episode of
Friends, but Leila wasn't a main star on
MAY. Paul Reiser did not guest star on
Friends.
NBC in particular does a lot of cross-over episodes, like
Law and Order SVU with
Chicago PD. I would think more than one Dick Wolf show should meet your conditions, since all of them have at least 2 main stars, Wolf reuses the same actors regularly, and the main actors often changed, especially on
Law and Order. Some Dick Wolf shows are NOT spinoffs; the
Law and Order, Chicago, and
FBI are all separate franchises.
I haven't given up yet.
