Perhaps the vapourous
Voldemort could have 'possessed' his own, younger body, and then the souls would re-unite in some way... If, then the two soul-fragments would live in the one frame, and share the control of the body, exerting an influence over the other self, or personality fragment.
There would be matters they agreed upon, but the older spirit would probably try to exert fuller control - however, as said, the diary's fragment would be more whole, more stable, and less (relatively) inured with evil, and possible thus stronger in some way.