Introduction can or can not be a tricky thing.
I've so many gliders that typically I don't bother with the long introduction processes anymore .. If one glider doesn't like another, sometimes they'll like a different one I put in the cage. Sometimes though, one of my gliders is fussy and stubborn and just doesn't want another glider invading their personal cage territory, which is understandable, to me. I've also had some gliders that, no matter what, no matter how many different gliders I tried, they never, ever allowed it to happen, this happening primarily with a much older widowed male I had.
I think it's best to do things like rotate their cage items to each other so they become used to their differing scents, like pouches or toys. You can put cages next to each other, but this may turn out to be a bad idea and they might display aggression being so close to each others territories, if they 'dislike' one another. In which case, you can block the side views with fleece, they'll still know something is there.
It's best to introduce gliders in a completely neutral environment, say, a bathtub (towel) with you inside, or a safe room .. a clean bonding tent is always good. A medium sized space is best, enough for them to be aware of the others presence but enough to keep their distance at first if they'd like. They should already have been well fed and you may have better results doing this in the daytime, when they're less high strung.
Occasionally I do offer food to gliders during introductions, but this can give particularity nasty results sometimes, especially if you're unaware if your glider is food aggressive towards others or not.
Squabbling is normal, I don't typically worry unless I see real biting or violent 'balling-up'. In which case I do have to step in, and if your hand isn't wrapped or gloved, you're going to get bit accidentally.
Take out both gliders every time you want to bond with them so they'll also be bonding with eachother.
It can take weeks for a proper introduction, but it's okay, and they'll eventually at the very least be able to tolerate each other. A lot of gliders are more than happy to get cagemates. All it takes is repeated processes. They'll eventually get used to another glider being around eachother.
And some gliders can get along completely, immediately! That's something I have happen all the time, I've been able to suddenly add several gliders into confirmed colonies without issues before.
Keep in mind that you're going to want to have some kind of health quarantine process if you're adopting a glider from a pet-store, I'd even recommend a vet visit, and they'll be a bit stressed during the first week of being inside your home. When the new glider is calm and confirmed to be healthy, start the introduction process.
I'd recommend a glider from a breeder. Gliders from breeders will be more likely to have been raised in multi-glider situations, colonies, trios, etc. They'll be more tolerant on average than a glider who you may not know the background of completely in a pet-store. Males are more tolerant on average, especially to females, so I'd recommend a neutered male if you can find one.
All introduction is going to take is patience, and don't let it stress you out more than the gliders.