Amel Super Maramu 2000 owners manual on page 16 have this:
12 V circuit: One only battery feeds the starters of both main engine and power generator engine. It is situated towards the front of the boat.
It is charged from two 12 V alternators one on each of these engines.
I do not if this is the same for SM. But you can easily check. Run engine and measure voltage on starter battery. Turn off the engine. Then run generator and measure voltage. It should be higher than when both engines are turned off.
#279 Bonne Anse in Portoroz