If you have 2 "s" in your rack, do you score statistically higher playing the extra "s" immediately or saving it?
As noted in the comments, it very much depends on the board. In general, S is useful for tacking on various plays your opponent makes - especially punishing them for thinking that stopping a word short of the TW row is safe. Two S tiles lets you possibly tack two words that end in the same row or column, and plenty of words (in English anyway) have double-s.
Given the general utility of pluralizing a word already on the board to create space for yourself - I would personally look to use each S in its own move, but only after confirming that I don't have any options to pluralize two words in one play - scoring three decently long words is pretty strong.