Making a co-op game (like the Fire Boy and Ice Girl series) is just a matter of having two player characters, one which moves generally with WASD, the other generally moving with arrows.
Here's a 5-part series on making a platformer (links in the "Updates" section for 2-5): http://www.kongregate.com/games/Abigayl/stencyl-2-0-tutorial-1-basicsThere is not going to be a direct walkthrough for what you are doing since it is something uncommon and is just a manipulation of a base game.