males will very often overbreed females if kept together
keeping females together depends on the females themself. given the option, the majority of my females choose to be near each other (and not just because they happen to want to be in the same place at the same time)
but females can also display dominance, so theres no set rules with anything
seeing as yours is a male, i really wouldn't recommend getting another to live in the same vivarium
he could overbreed a female and cause negetive effecst towards her health (there are a lot of problems related to females being over bred, not just the obvious weight loss)
and i would never suggest introduing another male to him. He's been living alone, introducing a male to him WILL result in them atleast displaying to each other, possible fights, bites, injuries, or at the very least, one male being suppressed which could cause problems with eating etc.
please, if you want another beardie, get another viv
if you want more than one more (in a new viv) then get females.
the majority of people who own bearded dragons say its acceptable to keep a trio in a 4x2 viv. There are people on either side of this fence, who say only one, and others who say more.
when it comes down to it, it's up to you to make a sensible judgment taking your animals interests into consideration