confusion about bahvior of female beardies

Question
We have 2 female beardies that are a year old.  One is a bit fat - we wondered if she might have non-fertile eggs so placed a tub of soil in their viv.  They both LOVE to dig in the soil, but have have since been displaying their beards frequently.    What I'm confused about is that 1 of the girls jumps on the other beardie and bites her neck.  It doesn't look like typical fighting, it looks like breeding.  I've checked to make sure we've got 2 girls, and though I'm no expert, they both appear to have the single bulge above the vent.  Also, when I pull them apart, the top beardie has interlocked her legs around the other and from looking at her vent, it seems like she's trying to breed.  I'm not sure if this is ok behavior, or if we should separate them.  Many thanks.

Answer
 This is a domination behavior females will do in the wild or if housed together.  The top beardie is trying to force her rival into an egg laying cycle so she will not be available to a male.  The other girl will lay infertile eggs as a result.  It's a survival behaviour, and an intimidation behaviour.  It's time to separate the girls.