Well an interesting hack (of a hack) was pirated / imported games couldn't be played because they didn't have a specific chip. They did have splitters so you took the chip signal from a genuine cart, and the other game plugged in the other port.
So you could have your camera and printer.
SNES => SNES Game Genie => Super Game Boy => Game Boy Game Genie => Game Boy Game. (Or even better, the Game Boy Camera or Printer)