It is never good to house a male and female together when young, since males tend to mature faster than females and will attempt to breed her, causing stress etc. Also, a young female (ie under a year old) can still become gravid and producing young when she herself is under a year, is very stressful and hard on her body, with a higher risk of complications.
Two females can sometimes be housed fine together, in a large enough cage (for two, I recommend no less than 5'x2' of floor space). However, there are always risks of the following:
One not thriving due to dominance of other, resulting in stress, not eating enough, not getting proper basking temps or UVB exposure, thus growing slower and eventually being out paced by the cage mate (and if the cage mate grows large enough, may even eat the smaller dragon).
Aggressive behaviour. The more dominate animal may constantly attack the other, ripping off toes, tail tips and even limbs, and eventually, can result in the death of the other.
Spread of diseases. If one dragon picks up a disease or parasites, both animals will have to be treated right away, as cross contamination will easily occur. Stress caused by conditions above, can lead ot flare up of coccidia and other internal parasites that normally exist in a small but balanced amount.
Even if the females get along perfectly with no dominance issues etc. there is still the risk that a female will suddenly decide she doesn't want her cage mate anymore and attack her. It only takes a few minutes for a dragon to be attacked and loose limbs or be killed.
All this being said, it is possible to house more than one dragon together, but it should not be done by the first time owner, and a second cage should be ready just in case.
To breed bearded dragons, introducing the male to the females cage for mating is fine, typically a few hours to a couple days is all that is needed to ensure proper fertilization for several clutches that year. After that it is best to keep the males and females separate. Of, if you have large enough space, a male and 2-3 females can be housed together all the time. But again, you have to watch for signs of aggression and separate the females or the male if it becomes necessary.
Two males should never be housed together. Once sexually mature they can fight aggressively. A large cage big enough for both to have their separate territories, may be possible to keep two males inside (pretty much a cage nearly twice as big as the minimum recommended floor space of 4'x2' (so 16 square feet of floor space for two males which would be 4'x4' or similar)
Females should not be bred under the age of 14 months, though 18 months or older is recommended before breeding. Males should be about 12 years old or older.
I also don't recommend breeding dragons unless it is something you have seriously looked into and have the spare money for all the housings/food needed for hatchlings before they are old enough to find new homes (at least 6-8 weeks old or about 6-8" in length). Selling them to some petstore or wholesaler isn't a good excuse to breed in my opinion, as not all of these will take animals from anybody off the street. Some required vet checks to ensure health etc. Also, most petstores already have plenty bearded dragons available to sell in their stores.
Breeding dragons is not cheap. Nor is there likely to be a profit in it, especially in the first year. Babies eat alot and a single female dragon is very prolific, capable of having as many as 5 clutches a season (average about 3-4 but I have heard of females having 6-7 clutches). Each clutch can have anywhere between 10-30 eggs (average being around 15-20). Hatchlings can also eat as much as 100 crickets a day, each.
So, if we take an average clutch number with an average clutch size and each hatchling eats an average amount of crickets each day...you can easily be spending $30 a day on crickets alone. (3 clutches of 15 eggs each with each baby, once it hatches of course, eating 50 crickets each, means 2250 crickets per day.)
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PHLdyPayne