[2022 note: This is a paper that I wrote for a technical writing class in college in 2012. It might also be interesting to other people as well, although there are probably better instructions elsewhere. I was reminded of it after seeing Tonkotsu's SGDQ reverse boss order run. Also the original title said "a glitch" instead of "the split-up glitch".]
Donkey Kong Country is a platform video game made by Rare for the Super Nintendo Entertainment System (SNES). One of its main distinguishing features is how it handles its two playable characters. Donkey Kong is an ape who is larger and stronger; Diddy Kong is a monkey who is smaller and more agile. Normally the player can switch between the characters within a level; however, if one character gets hit by an enemy, that character runs away and the player must continue with the other character. The character can be recovered if the player finds and breaks open a barrel with the letters DK on it. Normally if the inactive character is available, he'll follow behind the main character but not interact with the level. However, a glitch can cause the game to treat both characters as if they were the active character to some extent.
This set of instructions intends to explain how to perform this glitch. It is intended for people who are at least somewhat familiar with the game and have some interest in glitches. The glitch does not give any significant advantage in the game; it is mainly useful for entertainment.
[2022 note: I probably should have qualified this as "…any significant advantage that I know of". It looks like it can now be used in combination with other glitches to skip ahead in the game; I think this hadn't been discovered when I wrote this, but it's possible that I just wasn't aware of it. I still don't know how to use it to skip ahead. This page focuses on effects of the glitch that aren't useful for speedrunning or skipping ahead.]
These instructions assume that you have a way to play the game; this could be on the original console (system, controller, TV, and cartridge) or on the Wii's Virtual Console. Setting up the game is beyond the scope of this document.
I doubt this works on the Game Boy Color or Game Boy Advance versions.
Once the glitch is in effect, pressing a button on the controller will cause both monkeys to react as if they're active. For instance, Donkey and Diddy will move at different speeds. If Donkey and Diddy get too far apart, you can press Down and Y at the same time; this will cause Donkey to do a ground pound (which he does without moving) and will cause Diddy to do a cartwheel. You can also walk against a wall.
A major source of additional glitches once this is done involves the animal buddies. If you try to get both monkeys on an animal buddy, it will cause a clone of the animal to appear in a different color. Additional graphical glitches will happen as you move around the level, and it's possible to use this glitch to get animal buddies into different levels.
[2022 note: To get an animal buddy into a different level, you have to get both monkeys on an animal buddy and then drop one of them into a pit. I'm pretty sure it matters which one, but I don't remember which. Then enter a stage that normally has an animal buddy, or Reptile Rumble. On the version I had, this would always give you Rambi the Rhinoceros, but I think on some earlier versions it gives you whatever animal buddy you dropped into a pit.]
The game will return to normal if you exit the level in any way (win, lose a life, Start-Select) or if you enter a bonus level.
If the game freezes due to getting hit by an enemy, press Reset on your SNES. The game will not be damaged and no saved data will be lost.
Images from http://www.youtube.com/watch?v=7cY4oJEuPM4&feature=related and http://www.youtube.com/watch?v=MxI_4JWvrSc&feature=channel (this may also be where I first learned about this)
[2022 note: Those videos are titled "DKC Best Glitch Ever Part 1" and "…Part 2", uploaded by tjp7154 on Jul 11, 2007. When I wrote this, I had already known about this glitch long enough to have forgotten when I learned it, and back then I didn't pay that much attention to who uploaded the various videos I watched.]