Correction: It’s not two-player only, and you can play single-player if you want, with either controller port.

Also, this seems to be the very first revision of DABG, and there’s been several bugfixes and changes since then:
*Extra menu on title screen was removed and the level editor and credits are just moved to the main menu instead
*It now waits for you to unpress Start if you use Start on the pre-level screen to skip it, so you don’t start the level paused
*Credits screen now has website URLs on it
*All unofficial/illegal opcodes are replaced with official ones for better compatibility
*Fixed a bug where testing a level in the level editor after beating the game would just bring you back to the title
*Fixed a bug where enemies can’t climb over blocks from the right

Latest is always at http://novasquirrel.com/dl/DABG.zip