Back before I ever heard of Keyclone, I 2 and 3 boxed World of Kung Fu beta on one computer.
It's one of those game's that's free to play, but you're able to buy "Gold" pieces with your credit card, and this "Gold" can be used on exclusive items.
It's also possible to never pay a cent and be the top guild in the game (None of them had ever bought gold. They used ingame currency (Taels) they had farmed to trade for Gold.
Very fun game, easy to run even if you're doing it without a keyboard emulator. I'd recommend a spear user (AoE grinder) with a Fans (healer) or Hooks (great buffs and minor heals) partner to 2box. Anything past that would be gravy.
-Mc