The Handheld VHF radio should have a separate MMSI # for the following reasons;
1. It is very easy to change the descriptor information associated with the MMSI number. You just need to log in and edit the form. So if you use it on other boats, dingy while cruising, or a daysailer, or the fishing boat, it is easy to update.
2. In the remarks section, you can state this is a handheld VHF radio associated with a mothership MMSI # and its description. The USCG will see the description. Very important in an abandon ship distress situation. This is also why it so important to keep the information current.
3. If either radio is sold in the future you can cancel your registration of the MMSI # allowing the new owner to obtain the programmed MMSI# in the unit. Example; You sold your main vessel with the permanently mounted VHF radio, You can then cancel your registration so the new owner can sign up for it. You can take your handheld VHF radio with you to the next vessel, just edit the registration form. If you do not cancel the MMSI# registration the new owner will incur the cost of reprogramming the VHF.
4. If someone on the main vessel wants to contact you on the dingy using DSC they can do so using the individual call selection. Then you can have a conversation without hailing on VHF 16 or another channel. This is not possible if both radios have the same MMSI#. You can also just ping the LAT LON for the handheld to find out were the kids really are.