I haven't used watersheds yet (I'm a surveyor - we typically don't care about those), but I took a look. Offhand, I'd say it looks like bad news.
Since the Watershed Style is part of the surface style, I don't think you can have two different Watershed Styles on the same surface. You could create a child Surface Style that uses a different Watershed Style, but your surface could only use one of the two styles. So it looks to me like you're out of luck, unless anyone else who uses Watersheds regularly has found something.