the cage is big enough to house two aduld dragons, but I won't recommend putting in a male dragon. For one, she will need to be a few months older before breeding her. (15 months is mimimum, with 18 months better). ALso, you will hace to find a male adult dragon the same size of her. Having hte male in there will mean she will be under stress as he tries to mate with her every opportunity he gets and they could fight with one or more getting injuries. Not only that, you will have to be prepared to incubate, raise and find homes for the babies.
A second female may work fine, but again, she will have to be the same size as your current female. There is no guarentee they will get along and fighting can ensue, resulting in injuries to one or both dragons. Also a dominating female can stress out the other, preventing it from basking, eating or thriving.
In the long run, probably better to just keep the dragon alone and build an additional cage or more if you decide to have more than one bearded dragon in the house hold.
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PHLdyPayne