As a gerbil owner this is a constant dilemma for me aswell. One tends to always die much sooner than the another and it breaks my heart seeing the other be left alone. If I then get another one then the same thing is just going to happen again due to their age difference and if I get two there's like 50% chance they don't get along and now I have two terrariums with one gerbil in one and two in the another.

If anyone else is considering getting gerbils my advice would be to either get 3 males or go with a hamster instead since they live alone. I currently have 3 females but I almost had to separate them earlier due to fighting. This is less likely to happen with males.