Grown together as one creature.
Incorpsed is an amazing word. Especially since it has corpse in it.
I expect this is also where we get incorporated –
In the body.
Who leads who follows
Two minds as one
Two bodies as one
Two natures as one
Two natures in one nature
Of course – in this case it is clear who leads and who follows. It is the man who brings the horse to wondrous doings, not the other way round. It is the horse that is such a good follower that they seem two minds, two bodies, two natures in one.
I think rather that one has so dominated the other that his will is no longer visible.