Re: episodes that are part of two different TV series
The Star Trek TAS episode 'More Tribbles More Troubles' was released on TAS Complete Series Disc 1 and on TOS-R Season 2 Disc 5.
"More Tribbles More Troubles" isn't part of two shows. Just one, the animated Star Trek series. It's no more part of TOS than any of the other TAS episodes.
"Human beings worship the great god Santa, a creature with fearsome claws, and his wife Mary. And every Christmas Eve, the people of UK go to war with the country of Turkey. They then eat the Turkey people for Christmas dinner like savages."