My personal choice would be two males, but please don`t let that put you off taking on two existing females who are living together. I never kept female gerbils I always kept males. My first four boys had to be split up into pairs as two of the four de-clanned. When my numbers dropped, I brought in a nine week old baby male to pair up with one of my gorgeous, laid back little males and he was so gentle, they hit it off immediately, I almost cried! But Grub was a darling little gerbils with no dominant behaviour, so I knew he was an ideal candidate to introduce to a younger baby and it worked so well. Both lived together for the rest of their lives, but sadly, Grub injured a tooth one day while playing on the floor, he started to bar chew one of those multi-coloured hamster play pens and twanged his teeth. He was never the same health-wise after that and I lost him.
One of the main reasons I don`t like gerbils near bars.
Munchie, who was the baby gerbil I brought in lived alone after that, but he too somehow managed to injure his tooth and I felt he got an infection in the gum that got into his bloodstream because he went lethargic one day after being such a big, robust lad and he passed away.
Gerbils need careful observation while playing out of their tanks and kept away from any bars they can chew on or obstacles they can find trouble with! They are so nosey, that`s their problem! But they are just lovely and I miss my boys. x