My light is a 10.0 t5. There is a bar that goes on top of the screen so I couldn't put one long on on top. There are two 12' inch ones side by side separated by the bar.
Basically all T8's have to be mounted inside, along with T5 5.0's. While T5 10.0's can safely be mounted on top. The reason this is, is because the other lights, t8 10.0, t5 5.0 are not strong enough to go through the mesh while the t5 10.0's can.
I don't usually see 12'' T5s. It's fine to use them side by side. Are you using it in a fixture that has a reflector? If you are, then it can sit on top but if it's shining through the mesh I'd bring the basking area to within about 10'' of it. Those tops have a pretty fine mesh that intercepts lots of UVB. If it's not going through a mesh top then 12'' above the basking area is about right with a reflector. Without a reflector, the effective distance is much closer. Probably more like 8''. It's usually best to use a reflector to direct as much UVB downward as possible and make the most out of your bulbs.