Kerdi is good stuff, but like any typical shower drain, it is designed for 2". I'm not a pro, but I don't see any good reason why you'd need two drains with only two showerheads...people get buy with 4-5 or more with only one. I loaned my TDNA handbook to someone so I can't see if they require two because of the size of the room, not the quantity of the heads. Have you checked with the inspector? The plumber might be right on, or he could be blowing smoke.
Schluter talked about maybe making a trench drain, which might work for you. You might want to ask (I don't remember it in the catalog, but sometimes they have some things that aren't), but absolutely no guarantees.