I just bought the R2 Dual Extreme Moonlight (basically two tubes) for my 46g Bowfront (36" long). They only arrived yesterday so haven't had much time to experiment with positioning but one each side of the center brace about 12" from each end lights the tank nicely. I don't think one would be enough, would have to be off-center cos of the brace which would make the coverage uneven.
Trying to get pics, but I'm gonna have to play with my camera to get good pics with such low light.
One word of caution, if you order from R2 themselves, check they have them in stock, I tried 3 or 4 other places who all came back a day later saying they were backordered, R2 charged my credit card and then didn't send them for another two weeks cos they were backordered from them also. They ship from CA UPS Ground so it's another week for delivery.