bearded dragons are primarily solitary creatures (they don't like to be with other dragons). to get 2, you'd really need to have two viv's, or one really large viv (think tegu size, or around 8'x4'x4'). putting two dragons together can lead to aggression, toe and tail nips, malnourishment, etc.
the minimum size for a fully grown bearded dragon is approximately 4'x2'x2' (122cm x 61cm x 61cm) or 120 gallons, although they could be a bit less tall to around 18" (46cm). aquariums are alright to keep them in, but you'd be better off building your own if you have a little woodworking talent.
one thing that is IMPERATIVE to have is a good source of UVB lighting, mercury vapor bulbs are the best, but they are a bit on the expensive side, you can also go with the fluorescent tubes (don't go with less than a 10.0 though)
personally, i have mine (just one) in a 6'x2'x2' that i built. in reality, mine is a bit bigger than it needs to be because i believe my beardie has adeno and is rather small for his age.