Hello and thanks for researching this important question!
Gerbils, like other rodents, bond with their family members at a young age. They then set up territories that they guard wth their lives. It is easiest to adapt an older gerbil to a very young juvenile, and very difficult to adapt two older animals...even a male and female would fight.
Since gerbils are very sociable (unlike hamsters), "strangers" can be taught to get along together...this is most easily done with a split-cage method that exposes them to each others scents and behaviors without allowing them physical contact.
Here is a link to a very comprehensive gerbil site that gives details about this method and other ways of helping two unrelated animals bond. It is possible, though, that these two girls may need separate quarters for their lifetimes if they just do not take to each other (like people!):
http://members.nanc.com/~mhaines/gerbil.html#section2.2
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