I don't condone the usage of double Skadi (or any Servant for that matter).
I generally use double "power" supports when I just want to blow past a fight and continue the story, or I'm short on time and want to use my AP quickly. Say what you will, but it does do wonders for speeding things up. This in particular is a popular farming setup.
@All I guess "condone" is too strong of a word, sorry about that, but I really have no intention of telling anyone how to play the game (which is good, because inefficient as I'm, if people were to follow what I say, things would end very badly).