PG. I had the same issue as you and asked the same questions of Boat US. They said no 2 radios should have the same MMSI and had to create an exception in their database so I could register a second MMSI number under my profile. She said it was not unusual to do that. When I registered my portable I referenced my boat's name in the info and added in the associated notes that it is a portable assigned to my vessel. I also noted that if a DSC distress call was received from this number then there is a strong possibility of a MOB. The notes are supposed to travel with the MMSI info and you can only hope that they do. I hope this is helpful.