I just saw some bits of a documentary on a two-headed girl, but I wasn't paying attention so I can't comment.
Although I suspect both members of such an arrangement would have to cooperate with each other, thus giving them both to one player makes sense. Further, while they have to pursue the same goals simultaneously (athletic or intellectual are, in this case, mutually exclusive), each head could well have different skills or spellcasting classes.
So are they individual PCs, or are they one PC? If they're one PC, it could be reasoned that their unusual physical nature requires additional experience and training to gain a level, thus two heads that could independantly learn ten levels can only learn five, spending the rest of their time figuring out how to coordinate swordfighting when you've got two people in control of the hand.
Now here's an idea... conjoined triplets.