It is suggested to do a month-long quarantine, and then go about switching pouches, cage sets, cages, etc etc so they can get used to the other's scent. It's also suggested, after quarantine, to put the cages near each other, so they can also get used to seeing, hearing, and smelling the other.
After all of that, then you do the tub thing.
Some people don't do quarantine if they have gliders from a reputable breeder, and the new one is also from a reputable breeder, and they are 100% sure no one has any parasites or anything like that. Parasites are communicable and often take a couple of weeks to show up, hence the quarantine. Sometimes illness takes a while to show up, too.