With my gerbils (i know there're different) they were two sisters and had been together for nearly 4 years. We bought two new gerbils who were also sisters and asked the shop which would be the best way to introduce them. They reccomended the split cage method. We didn't even buy a proper cage or anything, just hitched up two walls of wire mesh in the current cage.
After a week, we swapped them round sides so they could get used to each others smells, and after 3, we removed the wire. My dad was on standby with a leather glove in case they went for each other. At first, Ola and Smudge fought a little (although I have heard that you're meant to introduce them in a neutral environment like another box so perhaps that was the problem) but after a while we had to leave to go out. When we came back they were all snuggled up together asleep.
Smudge (the older one who lost her sister) was given a new lease of life by having the two youngsters running around. She died at the old age of 6 and now we just have Heidi and Ola.
So I would say it should be fine if you do it carefully and ask the et shop first.