Since you don't have a reflector for it you'll have to mount it inside anyway. However you need it right over the basking spot not on the cool side. So unless that basking bulb is a MVB your going to have to re-arrange the lighting a bit.
Venus is right. You’ll want to arrange the lighting so that the two lights overlap at one point and the beardie receives UV light while basking (vs. having the UV light on one side of the cage and the basking light on the other).
I have a similar question. We're building an enclosure that is 48" wide x 22" deep x 28" tall and are planning to run two 22" florescents for UVB and light in general. Does it matter whether we run them across the top or the back of the enclosure? I was thinking the top would offer more coverage to the cage, but most of the suggestions I'm seeing talk about them mounted on the back wall.
It's best to mount them on top so they are directly above the basking site. Just set it up right next to the basking light so the most intense UVB is along with the highest heat over the basking zone. Are you planning on using two separate T5 fixtures or a dual fixture like this? http://www.lightyourreptiles.com/24...arcadia-12-and-6-5k-day-light-bulbs-included/
We have two separate fixtures running the length of the cage. My plan is to keep the newest bulb on the basking side, and when its potency runs out I'll move it to the cool side and install a new one on the basking side. I'll have to get some pictures posted of Spike's new home!