Alter Ego is a logic platformer by Shiru, Kulor and Denis Grachev. You have to switch between them the hero and his alter ego to clear levels. It is a bit similar to the game Binary Land.

This game is a remake of recent ZX Spectrum game of the same name. The original version created by Denis Grachev from RetroSouls, you can see video of it here and get it here. NES version got new graphics, original music by Kulor (composed for the game), and some other changes.

Interesting technical feature of this game is that it is written in C, with cc65 compiler. You can get source code here. More details about this and development process in general are available in notes.txt file provided with the game.

http://nesdev.parodius.com/bbs/viewtopic.php?t=7999