My new gerbil is sick, her twin sister died?

I bought a young female gerbil from petsmart to introduce to my other gerbil of 3 months. I used split cage method. Two days later the new one started obsessively licking her own and had diarrhea, she had her fur all puffed out and could only wobble by the end of the second day. She was dead the morning after. I went back to petsmart and used the exchange policy to purchase her identical twin. Its been a day since I bought her, she's had no contact with the older one, and she too is now licking her obsessively. I'm really paranoid she's going to die too and I don't know the cause. I've owned 10 gerbils and they all died of old age at 3 or 4 years. I have no idea what is ailing these new arrivals.