Unfortunately it doesn't work that way, but it's only stun, so he should be okay and normally the spirit will let go once his prey is down (at least, that's how I always give my order: 'take them down and then go to the next'). I hope I go before the mage, so I can stunbolt his ass. Then the spirit will dissapear as well, since it's not bound (if I recall correctly). Another option is to summon a new one and have them fight it out on the astral (you can have only 1 unbound spirit, so he can't take that one away from me). I'll think a bit about it until it's my action.