Closest thing I ever saw to that:
I think it was in "Monsterous Geographica: Forest" (or around here if not) that I read of a creature that was a swam conjoined back to back with a vulture, one was always asleep when the other was awake, the swan good, the vulture evil. The vulture could make a skill check to fly upside-down to make people looking up from below think it was the swan mode. Started from human twins who got transformed and magicked together (in a single spell/ritual). Then later their wives got involved in the situation and had the same thing happen to them, and thus a new race was born.
And I still think having it act on 3 different initiatives and so forth doesn't work so hot... having a mode change MIGHT... but still not quite as good.