Combining a couple of old threads
Hiyas,
OK, first time I visited this page (Thundercats messageboard, not IMDb in general!), I remember seeing a couple of threads to the gist of:
1. "They should have had a teaming-up episode"
2. "Jackalman's Rebellion was rubbish"
I remember the first time I watched Jackalman's Rebellion, I figured that the Thundercats and Mutants would have to team up to deal with Jackalman's army. Methinks they missed a trick here.
Actually, on the teaming up front, you could argue that "Shadowmaster" ticks that box, at least loosely, as Mumm-Ra does go to the aid of the Thundercats, or at least have Ma-Mutt help them indirectly (although this is more to do with the fact that he doesn't want to be usurped by the Shadowmaster). It's not "teaming up" in the most official cartoon sense, but it's still co-operation. "Key Of Thundera" doesn't count, as Lion-O only rescued Mumm-Ra because he didn't want him wandering round inside the book.