A double fixture light is most likely being driven by a single ballast capable of running 2 bulbs. If you rewired it so the full power was going to just one of those bulbs and added another ballast (also capable of running 2 bulbs but wired to one) you would have double power running each bulb.
You actually only get about 70% more light as effeciency does drop but as UCF said you are getting more power to each bulb. Believe me they are bright. In my planted 75 gallon I use 2 overdriven 32 watt and one standard 40 watt bulb and it is all the light I need. Check out my tank in my sig and see for yourself.
I can't say enough abouy the 55w kit from ahsupply. I just completed a retrofit of an All-Glass hood for my 29g and it looks great. Directions are fantastic also.
