I think it sounds simply wonderful!
Yes, a collar is a powerful symbol of ownership, but that does not mean that you cannot be owned by several Dom/mes.
Picture a family with one slave and two Masters, both training the slave, and a collar with two names on it..
It is a poly situation, yes, and that is why it does not mean that the symbol of ownership is "watered down". Just as you can love more than one deeply and sincerely, you can be owned by more than one also sincerely.
It is much more rare with men as Doms as far as I know, but it does exist.
Oh what a thought!