You both can't play at the same time on the same PS3. So technically, if you don't plan on playing at the same time, you only need one account. Since one account can have multiple characters, you each can make a character.

If you are anal and want different accounts for the same PS3, then yes you would have to buy a second copy of the game, and set up a second sub. Which, IMO, is just wasted money if you don't plan on playing at the same times.