The cage is big enough for two adult bearded dragons. However not all females will get along and there is also the chance a breeder makes a mistake on the sex of the dragon. Bearded dragons are not the easiest to sex under 6" long and I have doubts with any breeder who claims he can sex hatchlings under that size. At least not by using the typical one bump or two bumps method. So to make sure you buy females get them at least 10" long and check the sex yourself (never hurts to learn how to do it as it is very simple to do.)
It is best to set up the cage so there is a wide large basking spot where both dragons can fit side by side or have separate basking areas completely. Feeding should be done in separate bowls too.
Care must be taken as if one dragon starts to fail to thrive or is being out grown by the other you will have to separate to ensure both are getting their fair share of food. Though females in general do get along there are some who do not. You will always have to be ready to permanently separate your two dragons at any time during their lives. Even some who had no problems for several years could suddenly turn on each other.
Then there is the other problems...not knowing who is pooping regularly or left the super smelly runny poo (a sign of parasites) or if one does get sick from any ailment that is communicable, then you have to treat both even if only one shows signs.
I don't think it makes much difference if you buy two sisters or females from different breeders in whether they get along best or not. Only real difference is if you buy from separate breeders you will have to keep both quarantined for at least a month to ensure a disease in one doesn't infect the other. Though most breeders have healthy animals, some may not know their dragon is sick or it may end up catching something at the reptile show etc. Thus its best to isolate new animals to be on the safe side. Easier to treat one for mites or stress induced internal parasites than both in the long run.
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PHLdyPayne