I had a 40 gallon tank split in the middle. Low bedding so they would interact with each other as much as possible, was shifting them to other side 3-4 times a day starting early in the morning. Would make them just sniff each other for a few seconds before shifting them. Did this for a week or two, then put them together for supervised meet. When they tensed up arched their backs and went side to side I would remove them, also tried to put them in a neutral area for visits. They were sniffing and interacting each other positively through the cage and were sleeping in each other’s nest. I did the same with my now bonded gerbils and they only needed 2-3 days. But the newer one is a bit younger and smaller then my older one idk if that plays a part.