Interesting question. The short answer is the same MMSI. By doing so you will tie both radios to the boat name that you registered for the MMSI.
Couple of scenarios come to mind particularly for the portable that will make things murky down the road:
1. You take the portable with you on a buddies boat and have to use it in an emergency,
I guess you would have to clear things up in the MAYDAY message or in your follow up communications with the Coast Guard,
2. When you sell the boat, normally the fixed radio stays and the portable would go with the seller.
In the case of the fixed radio, the MMSI information is simply changed to reflect the new owner's name. But in the case of the portable, the MMSI would have to be changed (reprogrammed) to reflect the MMSI of your new boat.
Yikes!!