Each tender or craft associated with a parent vessel will need to be programmed with it's own unique MMSI number. Please be advised that the standard MMSI number range is 200000000-799999999, with 98xxxxxxx for associated vessels.
Using the same MMSI for multiple units will cause conflicts where it will either appear to jump from position to position, or just not be received at all if the transmissions clash. This will need to be issued by your local radio authority.