Squirt is my 2 inch RES. I was just wondering, if I were to buy another similar-sized turtle to add to the tank, would be better idea to do this earlier or later?
Earlier, and it should be ok for about a year. If one turns out to be a male, you'll probably have to separate. If you wait and find out you have a female, you can try adding another female.
However, both male and females can be aggressive, so be prepared for separation and possible injuries.