I just want to clear something up. I'm asking for examples of episodes that were released in two different Complete Season DVD collections. I know it's not really the same thing as being part of two different series, so perhaps I should've given this thread a better title.
By the way, I really hate the website poobala.com/crossoverlist.html. The guy considers the slightest cameo or name-drop to be a crossover and makes the ridiculous assertion that if two shows have one crossover they must exist in the same universe. IMO, there should be at least two crossovers.