That's a neat fixture!
There are different types of fluorescent tubes, they are labeled by T# to identify them.
If it is a T8 (the bulb will be 1" diameter) then you can use a reptisun 10.0 in one slot (the one on the basking side) and a daylight bulb (this is a term they would label on the box it's basically just a nice bright light) in the other side. You could also put a UVB (10.0 or 5.0) in the otherwise of you want. Using no uv or a lower output would be to give a gradient, similar to how we provide heat.
Just going off the picture though it looks like a T12 (1-1/2") which would only be good for lighting, I don't think they make T12 UVB anymore.