My Female Bearded Dragon is on a rampage..?

OK, so I have a female beardie thats about a year and 3 months. I have had her sense she was a baby and has be raised by herself untill about 3 months ago. I bought another female for conpainionship. I only did it because she has always been gentile and very friendly. Now the ne bearded is a female about a year and a half, very sweet and friendly also and very submissive. Now for the first month or so they have gotten along fine...baskin with eachother, eatin together as well as cuddling sometimes in the log. my problem is ..now my younger female is becomming agressive towards the older one and nippin ant her neck. i kno its prolly that time to start housing them in diffrent ta.nks, but can anybody enlighten me lil more on whats going on