I've tested it also on PBEM and it works.
llamabeast, I don't think you should prohibit this on your server b/c there are legit scenarios where players would want that, for example when subbing for a teammate in a team game, or if setting a game with each (or some) player/s controls more than one nation.
I think you may want to issue a warning to the game host stating which nations are played from same machine and let host decide whether its sanctioned or not.
Specifically If you prohibit this you'll make my life a bit harder, as I'll be forced to juggle the two keys I have
Edit, thejeff you're much quicker than me, you've four minutes on me
