XONOX Double Ender
During the Great Atari Boom in the early 80's, developers struggled to differentiate themselves. Activision and Imagic made good games, while companies like Mythicon shoveled crap to the masses behind a low price tag. XONOX, however, went a different route. If one game is good, then two games are better, so why not make one cartridge with two games on it?
XONOX carts were double the size of normal Atari carts, with a game at each end. This was largely due to the fact that you just can't fit Chuck Norris in a regular-sized Atari cartridge.
It's worth noting that the name "XONOX" is an ambigram, chosen because it would appear the same whichever end of the cartridge was plugged in. XONOX was actually a division of the TV record company K-Tel.