As combat in DOS2 is turn-based I'd say there isn't really a need for a 2nd DM to help in combat.
You are right, that for combat you won't need a 2nd DM, althoug it might be fun.
BUT: a 2nd DM would be extremely helpful. While one is controlling the party and telling the story the other one already is setting up the next improvised area (placing objects and so on). Multiple GMs would totally change a lot of the general capacity of the whole GM Mode imho.