Pet stores always tell you they can cohabitate so they can sell you more than one.
It always ends up with the same results.
Most reptiles are territorial, even at a young age.
The reason they always look so calm in pet stores is because
A. They don't have a proper uv light, and
B. The temps are too low
Plus they are underfed, so eventually they start biting at each other
Make sure you divide the tank at least. They will each need their own tank. A 40 gallon is the minimum size for 1 beardie.