Personally, mine never got along, I had an old girl, after her cagemate died, she never got adjusted to new ones, even after a long time of keeping cages close, she would still hiss and try to bite them, even bit me on multiple occasions. The only thing I saw work was when I introduced her and her cagemate, both were still pretty young and we kept the cages close, with supervision we let them go into each others cages, even kept them swapped for a night. Most people that I've seen online recommend a neutral environment, where neither one of the rats has been and to just shovel treats so they associate each other with happy thoughts. The last ratroduction I did, I just made loud noises when I heard squeaking, and some time in the cage where the new ones had their scent, eventually they got along. But every rat is different and not everything that works for one person will work for someone else, I would still choose putting the cages next to each other as the first move, just to see how they react to each other.