The pet shops tend to give bad information, so I wouldn't trust them with that. They keep them together because baby beardies can be kept together, but its best not to if you can avoid it.
I think he is just using the bulb that came with it maybe? I never really used the zilla bulbs much myself, so I don't know how good that one is.
Also, another thing To take into consideration is that the screen. I'm not sure if this is what you are doing, but if you have the
UVB bulb on the screen, it decreases the level of UVB, thereby needing you to put it closer to your beardies basking spot. Its better if it doesn't go through a filter.
As for keeping them together, if you don't want to get another tank, you can always just try splitting the tank in half with a divider so that they can have their own sides. Either way, if you already saw signs of bullying, I would definitely suggest separating them to prevent any fights or stress.
Its not that beardies can't interact at all, its just best to keep it more controlled.