The current cage you have is far too small even for a baby bearded dragon. A one foot cube sized cage is very difficult to heat properly etc.
Your adult cage doesn't have to be that tall, unless you plan to build two levels, with the lower level for one dragon and the upper level for the other dragon, this way both dragons are completely separated. It is recommened not to house dragons in the same cage together year around for many reasons. Dragons do not always get along with eachother, they are not very social at all. Thus, they can fight, with one or more dragons being injured, even killed. Also, one can intimidate the other, taking over the best basking spots, the bulk of the food etc. which usually results in one big dragon and one small dragon which typically doesn't survive very long. Also, if one catches something, the other will get it as well, meaning you have to treat both for any parasites, diseases etc which means more vet bills should something go wrong.
Other reasons against keeping more than one dragon per cage is difficulties between sexes. Two males, unless in a very very large cage (something along the lines of 8x2x2 feet) will almost deffinitely fight once both are sexually mature. Two females may get along but even females who have been together all their lives could suddenly decide they hate eachother and fight. If you have a male and female, you will wind up with the female being chased around by the male before she is old enough to safely have babies. Males are sexually mature as early as 6 months, and females are capable of producing eggs around 8 months of age, but some have been gravid earlier than that. Females should not be bred till they are at least 15 months old, many breeders recommend 18 months or older before breeding the female the first time. Males should be a year old, to make sure all hteir muscles are fully developed. Younger males can often suffer from prolapses, which basically means part or all of the hemipenis extrudes from the vent, usually because the male is unable to retract them properly.
Aside from being too young, having a male and female living together can mean alot of stress on the female. The male will try and breed her as much as he is able, resulting in a female who never gets a break, is often stressed from mating and being chased, doesn't eat as well etc. On top of all that, she now has a clutch to bear, which takes it's own toll on the body. As dragons are able to have multiple clutches per breeding season, even with one single mating, the production of eggs and shelling them takes a toll on her calcium reserves.
Besides the stress of egg laying on the female, you will have the sudden responsibility of incubating or destroying these eggs and making sure your female gets all the nutrition she needs, w hich means extra protein and calcium.
So it is best to house dragons separated. If you do want to breed and have a male and female, just introduce the male when the female is old enough just long enough for the male to copulate with the female a few times and then take him out.
The minimum cage dimensions for a single adult bearded dragon should be 4'x2'x2'. For each additional dragon, if you decide to house them together despite the risks and problems it can cause, add another foot to the length or width. More is better. The height isn't as important as bearded dragons are only semi-arboreal. They do not need alot of climbing space but some branches to climb up on are fine. Two feet of height is more than enough height for a bearded dragon cage. Keep in mind you want the dragon to get within 8" of the UVB light (if a florescent tube) and 18" or further from a Mercury Vapour Bulb uvb light.
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PHLdyPayne