I don’t follow Nickelodeon but I just heard about the TV series Sam & Cat. It apparently took two supporting characters from two previous series - iCarly and Victorious - and brought them together for a new series.
Spinoffs where a supporting character is given a new series are fairly common. But has there ever been any previous series where two unrelated shows had a joint spinoff?
In the world of daytime soaps, characters and settings have been spun around in a dizzying set of reboots. Another World had a spinoff called Somerset, which was canceled and replaced by a soap called Lovers and Friends. That was retooled into something called For Better, For Worse while still retaining the character of Amy Gifford from Another World.
And then there’s the mysterious case of Earle Stanley Gardner. When CBS decided to bring his* Perry Mason* series from radio to TV, they had the brilliant idea of spinning off tsome elements and the general crime motif into a daytime soap opera. The Edge of Night was originally designed to be a full-fledged spinoff, but Gardner had creative differences about the direction of the Perry Mason character. TEON used a different character, but he was played by the actor who played Perry Mason on radio, and both of the shows lasted for years, although they never acknowledged each other.
I’ve got a really old example: The Merry Wives of Windsor, which features both Falstaff and Mistress Quickly. Since the former originated from the histories while the latter is from the comedies, I think it’s safe to say they were originally unrelated continuities.
This isn’t quite the same thing, but the character of Tenspeed Turner, played by Ben Vereen, who starred in the short-lived 1980 TV show Tenspeed and Brownshoe, showed up as a major character in the 1987-1988 TV show J. J. Starbuck. (Really what happened here was that the program J. J. Starbuck wasn’t getting good enough ratings, which made it necessary to bring in the character of Tenspeed from the earlier show, so it was sort of combining pieces of both shows.) But if you count that, perhaps you should count the character of John Munch, played by Richard Belzer, who showed up in many shows.