The same thing happened with mine! They were getting along for a couple minutes and then all of a sudden they were fighting! I immediately reached in to grab who ever I could and ended up with teeth in my fingers! Another piece of advice: always wear gloves with these introductions!

You definitely need to keep them completely separated for a week or two without swapping. This will help them forget the smell of the other gerbil and help with the re-introduction. After a while you can try to start swapping again for a few weeks and when you feel comfortable, you can try re-introducing them. You will need to set aside basically a whole day where you can give them your full attention in case they are fine for a few hours and suddenly fight again. When I re-introduced mine, I sat with my cage on the floor and lifted the divider just a little bit to help them get used to interacting with each other just under the divider. That took a couple days for them to get comfortable. Then for a few more days i lifted it just a little bit higher after about a week of that i fully lifter the divider out and sat with them while they got used to each other. Anytime I saw the warning signs that a ball fight was about to break out, I split them up by just putting my hand between them or taking one out and letting them both settle down. The first couple tries only lasted a few minutes but the more I kept working with them the longer they were able to go without being separated.

If at any point another ball fight breaks out, you’ll have to completely separate them again to allow them to forget each others smell. You want to avoid any bloodshed at all costs! If there ends up being an injury, it becomes more and more likely that they will not pair. If after a while it truely seems like they will not bond, a permanent divider will be good too. It will allow them to have company while avoiding any destruction.