It really all depends on what dragons you end up buying and what age/sex they are. Since you are interested in our pair, I'll answer the questions regarding those.
First off, these two dragons are related (same parents) so therefore I would not keep them together once they reach 4 or 5 months old.. at that age the male will most likely start showing breeding behaviors and you do not want him to breed a dragon that is related to him (and such a young age as well). So therefore they would need to be separated. For that matter, any male/female pair should be separated at that age because you don't want them breeding to early.
I don't know what size a 50gallon is, but you don't want to put small dragons in a large tank. A cage the size of 22" long by 15" wide x 12" is what they are in right now... putting them in a much larger cage will stess them out.. smaller dragons do better in smaller cages until they grow some.
About the 6 x 3.5 x 3... yea you could divide it as long as you provided the correct setup on both sides and each side had access to a cool zone. You don't want one cage really hot and the other cage cold because of heating problems.
My advice to you is if you want to get two dragons, get two females and raise them up together.. that way you don't have to worry about them breeding and you can keep them together a lot easier.. though I still think it's better to separate them. If you want to buy a male and female, either buy one dragon from two different breeders, or buy two from a breeder who offers different bloodlines.
Hope that helps.
brandon
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Dragons by Nature