EDIT: Unfortunately, this glitch was fixed in a recent update.

Only works in co-op as far as I know.

An odd glitch in Spelunky that messes with the level generation. The best way to do this is to set both players to the same input from the controls menu. Also, the reason I walk against some walls like a total idiot is to make sure that both characters are exactly on top of each other. (giggity)

Another cool thing I forgot to mention is that "rushing water" levels also carry on their effects. Try combining that with the black market for some very strange generation.