Gerbils Fighting!

My gerbils have lived peacefully together since 2006. They are sisters and I got them both as infants. They are housed in a fish tank with a tiered wire cage above.
   
They have recently started chasing each other up and down the three levels of the cage. I have also found significant bite marks on thier tails, face, and rumps. The wounds were bleeding and I put bacitracin on them. However, they do still sleep together in their nest.
   
Should I seperate them or keep them together? If I keep them together, how do I keep them from obtaining any further injuries? Some websites say to keep them together because they are social creatures. Other websites say to seperate them because the dominant gerbil will kill the other one.
   
Thank you for any advice you can provide.

Separate them! The relationship has broken down & they hate each other now. Divide the cage & give each gerbil half of it with duplicate furniture & equipment. Then they can swear at each other safely.
   
Link: http://agsgerbils.org/Learn/Gerbil_Care_Handbook/ index.php,
   
heading "Split Cage Method".