Disney will not allow you to change the "lead name" on a reservation. Any other person in the room can be removed and replaced with another name but the lead name must stay the same. So, if you make a room reservation for yourself and then another for your husband, you would not be allowed to substitute another name for his later on.
What this means is that his MB will open the second room. His charging privileges will be associated with the second room. But, it doesn't necessarily mean that he has to sleep in that other room. He can sleep in your room as long as you do not exceed the room occupancy limits (which might necessitate moving another person from your room into his room). You also cannot leave unattended minors overnight in the second room unless there is a connecting door between your rooms.