Sure get a second beardie BUT make sure it has it's own enclosure and that you never let them be together.Two beardies = twice the fun watching them and interacting with them.
Is good that you are upgrading the dragon you have to a larger tank, he'll appreciate the extra space once he settles into it.
As already mentioned , beardies are better off living in their own individual tanks. It was not mentioned but if there is a considerable size difference between the two beardies, that the bigger beardie if he's hungry is apt to view the little one as a snack.
Even housing similar sized beardies together is very risky (even if they are siblings), they very quickly establish the pecking order and so the dominant beardie will thrive because it gets the lion's share of the food , the best access to basking spots, the best place to nap, and the other dragon will fail to thrive / grow and will be under continuous stress , this is without obvious aggression and bullying.
The apparent peace wont last , eventually the bigger dragon will start chasing and biting the subservient dragon , and this can because bearded dragons are territorial (turn very bloody and the dominant dragon will likely kill the other if it can't excape).