Gerbils are social animals that become depressed if they are alone too long. The catch is that gerbils are extremely territorial and will do anything to kill an intruder.

The only safe way to introduce two unknown gerbils together is to make a cohabitation by splitcage. To put it simply: you divide a vivarium in two by placing a grid partition in the center. The two gerbils can see each other, smell and interact, but they can't touch each other, and therefore get hurt.

Here is a list of possible combinations.

 

The easy ones : Two 8 weeks old (or less) pups

The possible ones that might require time : An adult with one or two 8-10 weeks old pup, an adult male with an adult female (preferably in heat), two adults
The difficults, almost impossible ones : Two 1 year or older females

An important thing is to remember that you should NEVER attempt to introduce a gerbil to an already established clan. Clan means two gerbils more than 10 weeks old that are together. Attempting to add a new gerbil will cause an imbalance in the hierarchy and will inevitably end up in battle.