The egg crate dividing the tank worked great for us. We added our purple into our 100g maybe three months after the yellow tang, who was very sure he was boss of the tank by then. They scrapped pretty badly. Had to move a couple of pieces of rock in order to get the egg crate in there, then left it for a week. When we finally pulled it out, there was peace on earth.

Funny enough, we could not wedge the egg crate perfectly no matter how hard we tried, there was always a tiny gap somewhere. Every other fish in that tank found its way between the two sides, but the tangs could not, so they were the only ones really on restriction.