Oh, the parasitic twin idea isn't a bad one, the idea is that a spark splits but one of the resulting twins doesn't develop properly, and may even be missed when the larger twin spark is placed into a body. Some time later, the smaller twin spark starts causing problems for the bigger one at which time a check of the larger spark uncovers the smaller one, surviving by drawing power from it's larger twin.
Because the parasitic twin has not developed properly, it will never be able to survive as a normal mech, probably not even be fully sentient, so it gets put into a small body and due to the twin bond acts as it's larger twin's drone.

So, the idea I have on it is that some time in his early years, Orion has spark problems that turn out to be an improperly formed twin that has been drawing on his spark. The parasitic twin is removed and turned into Barrelroller, Orion's drone who later becomes Roller, Optimus Prime's drone.