Reddit reviews Apple Flavored Ivermectin Paste by Durvet, Inc. Animal Health Products [Pet Supplies]

Your rats might just have mites and the b/w rat is really scratching and digging at herself. If they were really fighting then they wouldn't be sleeping together and the fights would be constant. Sounds like what you described is just playing. You could purchase some ivermectin from a tractor supply store. It's less than $20 so it's worth trying. Just give a small amount (size of a grain of rice) once a week to both rats for 3-4 weeks.

http://www.amazon.com/gp/aw/d/B00II71QN2/ref=mp_s_a_1_12?qid=1420130566&sr=8-12&pi=AC_SX110_SY165_QL70

Here is the desktop version of your link