They shouldn't kill each other, but there will be some territorial stress related issues with two SCS in the same tank. Unless they happen to be opposite sex, in that case they would pair, but I don't think there is a reliable method of telling.
Curiousity quetion: how much did the pair of shrimp roam? Did they roam the tank as frequently as the single one? I ask since mine just recently passed (had him for years) and I'm debating whether or not I want a single active one or a stastionary pair as a replacement.