Rights of a Non-Custodial Parent?

My husband's children will be staying with us during the summer for 42 days. They both have joint-custody, and both have the right to make medical decisions but not involving and invasive procedure (any surgery would require both their consent). However we thought it would be smart idea to have a medical release form, for me, because 1.) I am their stepmother 2.) The children will be with me the majority of the time in my care, with the only family car that carries their carseat's and in the event of an emergency whether being Major or Minor, I am the one able to seek immediate medical attention most quickly. We offered the medical consent to the children's mother (custodial parent) and she disagreed to consent to any form. Since my husband has joint custody, does he have the right to have a Medical Release form that does not require him to have her permission or signature? Basically; does he have the right to designate me to make medical decisions for him, in his absence? since he has his own rights as a father? Or do is that something that both parents would have to sign considering they are both living, both have joint custody and the children primarily live with her.