Playing Two Character at once.

It something I was wondering if anyone ever did in any roleplaying thing they ever did, Play two characters at once? I remember when I was doing free form forum roleplaying I played like digimon which of course since both the tamer and the digimon are sapient beings that can speak the same tongue meant I had to play both. I not sure if I really did a good job (probably not since digimon roleplay I remembered being a mary sue because I was young and stupid...) The reason I am asking an idea I had floating around for a tabletop game that would allow it was playing a person who was normally a fusion of two beings, but could be separated for plot reasons.



So has anyone ever played two characters at once or known someone who played two at once? Can you sort of give tips on doing such a thing without it just turning one player into a chess piece to moved on the board rather then a real charecter?

Playing Two Character at once.