For the most part it is recommended not to house bearded dragons together at all. Though there are behaviors missed when dragons are housed separately, for the most part, it is not recommended, especially by people new to keeping dragons and/or have limited space or funds to house them individually should multiple dragons not be compatible.
A single adult dragon needs at least 4'x2' of floor space in their cage. Some say you can go smaller but bigger is always better for these reptiles. Thus you need a cage at least 5'x2' to house two dragons, with larger being better. 5'x3' would be a good size for two dragons.
However, male/female pairs will breed. This does put more stress on the female as she ends up having multiple clutches of eggs not to mention you have to decide if you will incubate these eggs, or destroy them. It may sound fun to breed them just once...but to be honest, the amount of work involved, its better to buy a pair of leopard geckos or crested geckos and breed them. Cheaper in the long run and the 'breeding, hatching, raising' experience is the same, except with geckos, you only have maybe 10 babies to deal with in a breeding season, instead of anywhere between 30-100 or more eggs from a single female in a breeding season.
Two females can stay fairly peacefully together but there is always a risk. Even females who have been perfectly fine together for several years can suddenly fight and one or both ends up with missing toes or limbs or worse.
There is also the risks of one dominating the other so badly it prevents the cage mate from accessing the basking area and food. Thus you end up having one dragon thrive while the other fails to thrive, is always hiding etc. This can happen with babies and adults.
The spread of disease and parasites between cage mates is greater. If one is sick with parasites or some disease, the other most likely is too. Thus both have to be treated.
Thus it is best for new owners of bearded dragons to stick with housing them individually till they know alot more and understand dragon behavior and what to look for to know when a dragon is feeling stressed, isn't eating as much as the other etc.
As you have been asking quite a few questions about bearded dragons, I suggest reading the following links, which do give more information about various aspects of Bearded dragon Care:
www.beautifuldragons.503xtreme.com/home.html
(lots of great info on the above page, including a nutrition chart for what greens to feed, common Dragon diseases, etc.)
www.anapsid.org/mbd2.html (more info on MBD)
www.uvguide.co.uk/index.htm (all the info on UVB lighting you could care to know...including the dangers of the compact UVB florescent bulbs)
www.dachiu.com/beardeddragoncare/caresheets.html (good articles here on bearded dragon care and other info. just move the mouse pointer over the beginning of the headings, to see the links)
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PHLdyPayne