Like Tarasbaby said you want to be sure that you only house females together! If you do infact introduce another gecko with the one you already have I would reccommend changing up the tank setup just so its a new environment for both geckos! Doing this doesnt give the the gecko you already have an "advantage" over the other one and essentially puts them both in a brand new environment!
Ultimately it is best to get them at the same time but either way works! Be SURE they're both females though and seperate them immediately if you start to notice any abuse or bullying going on!