I'd like to add the key words in older gliders should be, Slow Introductions. Since both are over a year old, they make take longer to accept eachother than say a pair of joeys. All the tips you got above are great. When I introduce gliders it's usually after I keep the newest glider alone for a few weeks. This gives them time to adjust to their new home and glider parent before having to meet another glider, and if the new gliders is sick, you might not know at first.
Then, place cages close to eachother, 4-5 inches appart, that way they can hear, see, and smell eachother but not touch. After a week or so, I switch thier
pouches while they are out playing, so the are actually sleeping in the others bed that same night. I take out any other sleeping area so they have to sleep in the bed that smells like the other glider.
You can try carring them during the day in seperate
pouches so they can not touch eachother, but will be able to get the feel of eachother.
Then a few days later, introduce them in a controlled area (like a bathroom ). I take both
pouches out of their cages, take some treats, go into the bathroom and let them come out on their own. Offer treats, supervise thier interactions, etc. Let them greet eachother on their own, just don't take two gliders out and say "here you go, your new friend" they might want to move slower than you. You should expect to hear some crabby, hissing and they might play tag, touch eachother then run, back and forth. That is all fine. Just don't let them fight, if that happens, seperate them and try again the next day.
Lastly, when I feel all is fine, I move a sleeping glider in it's pouch to the cage they will ultimately stay in, clip it on to the cage. So both gliders will be asleep in thier own pouch. Then when they wake up, watch them to make sure all is well. You'll most likely find them both sleeping together that night.
I have not had any gliders that absolutely refuse a new friend. But I know it does happen, so just go slow and be patient.
Good luck to you.