Put one on each side.
I mounted my MH pendants and actinics inside a larger wooden pendant and suspended it over the tank. I like it better because I don't have to mess with the lights or canopy to work in the tank. It also helps a lot with the heat - doesn't trap it in like a canopy does.
The 3 sided skirt like Tom suggested will work too. That was my original plan, but I decided I liked the accessibility I had before with just pendants.