There are some really cool roleplaying opportunities that emerge from this idea. Maybe it's been done, but I would love to see an RPG mechanic that could actually model disagreements among parts of one being. Maybe two players operate different halves of "one" character...? A skilled GM, with sufficient commitment to roleplaying from his or her players, could probably set the goals of the halves at odds with one another.
This can even be expanded beyond two-headed examples. At work, I study a bryozoan (or moss animal), a colonial invertebrate. It's brain-warping to think about what it could be like to operate as an individual in a true colonial organism. The bryozoologist Judith Winston delivered a very nice lecture on this sort of thinking entitled Life in the Colonies: Learning the Alien Ways of Colonial Organisms. The entire article probably only interests scientists in very specific fields, but she does describe how her interest in SF influenced her appreciation for such an alien lifestyle:
Right here on Planet Ocean were creatures as bizarre in structure and unique in life-history patterns as any I had read about in science fiction. Many of them were marine invertebrates with clonal or colonial life histories, including bryozoans, the group on which I eventually focused my research.
I suppose people will often play a troupe of characters in RPGs...it would be interesting to see what would happen if these characters could be linked as a unit and presented with situations (and rules) that require a balance of altruism and selfishness.