when both those games came out I searched fr co-op mods, dude it will only cause you frustration, there are so many people making mods for games, and you never know what you gonna get cause its not organisations making the mods, its just groups of people.
I downloaded a co-op mod for doom that was 200kb in size, it made it co-op, but it was so buggy, my friend playing with me dissapear, and the game would freeze.
Its not worth it. but PC gamer magazine usually lists all the awesome mods for games, its at the end of the magazine, its called after-life.