Back at the dawn of the
Nintendo Entertainment System gamers could not get enough of the ports of the
arcade classics
Donkey Kong and
Donkey Kong Junior and, as more and more people snapped up the cartridges, these games became increasingly difficult to find. How did
Nintendo meet the demand? They combined the two games into a single cartridge, and in 1986
Donkey Kong Classics was born.
Upon powering up the NES players are given a menu where they can choose between single player or multiplayer version of the games (alternating multiplayer, mind you). There are no new enhancements to the games, nor are there hidden surprises. Nintendo simply took the two games and stuck them on the same cartridge, much as how Super Mario Brothers and Duck Hunt share a game pak.
The game pak itself is fairly hard to come by these days in most places, but the actual games contained on the cartridge live on in a number of ports on different platforms. Donkey Kong appears in Donkey Kong 64, the Game Boy title Donkey Kong, and the Famicom Mini game pak of the same name, for example, and Donkey Kong Junior pops up in both Game and Watch Gallery 3 and Game and Watch Gallery 4, albeit in slightly different variations. If you want the original cartridge I'd advise checking used game shops and online auctions, but in all honesty you'll have better luck acquiring one of the modern day ports.
TheBooBooKitty tells me that an aftermarket unauthorized kit pulls the same "combination of games" trick at the arcades with Double Donkey Kong.
References:
Playing the original games
http://www.GameFAQs.com