back in the day of 2D scrollers there was quite a few arcade games that you could do that on one keyboard. But I don't know of any modern games that would let you do that.
I'd refer you to arcade and old console emulators (google for MaMe, kawaks, gens)
the games for those are old and very old, but you'd be able to find quite a few coop ones that you can play using one keyboard.
To give you a few titles to look for: knights of the round table, golden axe series, armored core, streets or rage/bare knuckle, mortal kombat (older games), etc
Theres a lot, specially emulated games for PS and PS2 cause you can modify the controls in the keyboard. There are also newer PC games that can be played on a single PC though its not coop. Games like Street Fighter 4, Split Second can be played by 2 players using the keyboard alone. Sports games line NBA 2k12, Fifa 12 can be played using the keyboard and gamepad.