Tex Gal

I have a 125g and do use 2- 36" lights. One on each side of the tank. The only drawback to this is that the middle section is a little darker than the sides. I just put lower light plants there and it works fine. You could upgrade your lights and use just one for now. It would work. You just have to decide how much light you will eventually want on your 125G and if it's the same, then I'd say go ahead with it.

Ezekial

I've seen a 36" light that is 96 watts. That would give me roughly 1.5 W/gallon on the 58 gallon and two of those (192 watts) would give me roughly 1.5 W/gallon on the 125 so that would work. I think 1.5 W/gallon is the highest I want to go, I don't need a ton.