First you take them to the vets to see if they have any parasites. Then you quarantine them for a month so you will need two cages. During that month you can scent swap. Take their pouches they are sleeping in and swap them and the gliders will get used to each other's scents. I have moved my sugar gliders into the same room now that I'm only waiting on getting the male fixed. They can both see and hear each other and that has them getting familiar with each other.
DO NOT JUST STICK THEM IN A CAGE TOGETHER. That could easily result in fighting due to territory and a new intruder as well as if one of the gliders has a parasite or sickness then you will need to treat both.
But good news if you follow getting a second cage and getting both a health check up as well as the month quarantine and scent swapping and they are both lone gliders. Most of the time the introduction will be successful. In the gliderpedia there is a section on introducing sugar gliders.