I just had to do this - made total sense! I updated a 55 watt CF (coily tube bulb) full-spectrum to a 105 watt CF full spectrum rather than go to a second bulb (just re-positioned the bulb). Here's the setup

it's the bulb in the front. The two nearest the windows are still 55 watt bulbs. I measured over 600 foot-candles near the screen where the plants set. That's more than a doubling of light. Here's the source for the bulbs (pretty cheap for what they are)
105 Watt Compact Fluorescent I purchased the "full-spectrum bulbs, as they are best for plants.