I agree! I have been in a band with two singers and it didn't really work out well. Unless the guys are really cool with each other, you get some guys jumping in to other guys parts trying to steal thunder and the ego trips go a little too far. If they are cool with each other, I don't see any problem and things should probably work out well. I think the fact that they need two singers just compliments Christian's talent. When I saw them live he had no problem getting through all his parts.
I thought Sikth pulled off the two vocalist thing pretty well. But they would go back and forth between almost every line it seemed, so there wasn't much downtime for either of them. For one guy to do all their stuff he'd probably have to learn inward singing, haha.
Two vocalists with Scar Symmetry seems a bit weird, probably just because the lines were written for one God of a singer, as opposed to two mere mortals. For the band to be able to find ANOTHER single dude that could pull off Christian's stuff, they'd have to be the luckiest dudes on Earth, so I think the two singer approach is justified.
Scarve were great with two vocalists as well. I see nothing wrong with it. I'm excited for the future of Scar Symmetry and I wish the guys the best of luck!