From the Boat US MMSI FAQ page:
I have a handheld radio on board my boat. Should it have it's own MMSI number?
If you plan to use the handheld on other boats, you might want a separate MMSI number so that you can update the registration according to which boat it is currently on. If you plan to use it only on one boat, you can use the same MMSI number as the fixed radio.
I have a VHF Handheld with DSC capability. Can I register it for an MMSI through your
website?
If the handheld is for Marine use, yes; however you would need to keep the registration updated according to the specifics of which boat it is being used on. Reason being, the USCG could be misled in a distress situation thinking a distress came from a boat other than the one actually involved and could end up dispatching the wrong type of rescue unit or calling the wrong emergency contact in any particular situation. Also, it would be inappropriate to register handheld VHF if the intention is for over land use such as hiking. The Coast Guard will not be the appropriate response for this type of distress. Use of Marine VHF Radios on land is prohibited.
http://www.boatus.com/mmsi/MM3.htm#q23