In addition to Hat Trick, Tetris Cocktail, and Space Invaders II, try Hot Shots Tennis, Lazer Command, Astro Laser, M-4, Power Play
Leave the orientation 'sideways' and remap controls for Armor Battle, Windjammers, Alien Syndrome, Warrior, Blockade, Wizard of Wor, Demolition Derby, Demon, Gauntlet 1 and 2, Ripoff, Tank Force
Sprint, Warlords, Sprint/Championship Sprint, Offroad, Indy Heat, Tournament Table if you have dual spinners.
If you are running a relatively new version of mame, go to video settings within a game and select cocktail mode to get zounds of horizontal simultaneous 2-player games going.