You should consider neutering your two gliders before adding any additional friends for them. Adding another intact male would probably results in fighting for dominance.
You might consider getting them neutered then add two females that are about the same age.
Keep in mind you will need an additional cage for the 30 day quarantine (no contact, in another room) Introductions can be a pain but if you let the gliders determine when they are ready to be together it will all work out.
I just added a female to a trio that came to me - 2 males and one female. I wanted to balance out the family.
I also have a larger colony of 6 gliders 2 males and 4 females.
Colonies are fun to watch. Each glider has his or her own personality and they seem to have fun little games they play together.