I wouldn't say you were "wrong", by any means. Many people do house females together, myself included, with no problems, as you did up til now. But every beardie has its own individual needs and temperament, and some just don't have any interest in tolerating shared territory. YOu see this in any animal, certainly not just dragons. Sounds like one, or even both, of your girls may be one of these. She may just be coming into sexual maturity (BTW, are you positive you indeed have two females?), and be a more inherently dominant animal.
They *may* be able to work it out between them, but again, the subordinant female cannot strike out on her own for territory, which she would be able to do in the wild, if the dominant female does not allow her access to prime basking spots, space or food. And, unless one rapidly cedes dominance, they may injure each other in the struggle.
This is just a temperament issue, not one in which you have done anything wrong. Some animals of all species, particularly females, are more laid back about shared territory than others. If one or both are nesting, that too may be a contributing factor. When dealing with any animal, there are always variations in temperament and behavior that have to be considered, and what might indeed work "most" of the time can't be guaranteed to work in every case. Providing separate housing is going to be a pain, there's no doubt about that, and I am sorry that in your particular dragons' case it didn't work, or won't work for now.
A twenty as a temporary measure may get you through for a few days, but won't be healthy in the long term. You will of course, want to be absolutely certain that the mites have been eradicated before housing anybody else in the cage (or even in the same room). I personally would not risk it. As a temporary solution, if your dragons' current cage is large enough, you could erect some sort of barrier in the middle and provide two basking spots and caves, one on each side. This would minimize both the mite risk and the stress of moving twice (once now and again into another larger cage when you get one). Of course make sure each girl has access to warm and cooler spots.
