Boosting wattage in a twin hood 55 gallon

2 things you can do, you can go to www.ahsupply.com and retrofit that hood with a bright kit. Its really easy to do.

2nd thing you can do is something im not as familiar with which is called "overdriving" your normal output bulbs which basically uses a stronger ballast to pump more lumens into each bulb. I believe the plant geeks refer to it as "ODNO." From what ive read its pretty safe but you just lose some bulb life. Try googling it.

whats the exact length of your tank?

Unless you plan on a very low light 55, with a small selection of plants that will grow well, you will want to spend a bit more and get either a 2X55, or 2X65 retro-fit kit for it. I use the 2X65's in my 55's and am very happy with what they have the ability to grow.
A 55 is a relatively deep tank and you need the penetrating power of the 65 or 55 watt/CF to get to the lower parts of the plants, IMO.
If you have the standard boxes on top, that hold your 15watt tubes, the 2X55 watt set will probably fit nicely inside them and you won't believe the difference in light.
Call this number (AH Supply) and ask for Kim: 1 888 859-5106
Before calling him measure the inside dimensions of your boxes and have them ready so he'll know what will fit and what won't.

Len

I think part of what needs to be considered is what you are trying to accomplish and how much you are willing to spend. Djlen's idea will give you ideal lighting for most any sort of plant, but may also require a CO2 setup to avoid algae. At the same time the 1 x 13 and 2 x 13 kits aren't going to get you far enough. Another option would be a replacing your 24" light fixtures with a single 48" flourescent one. A 2 tube t-12 setup would put you at 80 watts or a 3 tube t-8 would give you 96 watts. Both should be decent for a low light planted tank, though I'd lean towards the 3 t-8s. If I'm thinking right you should be able to get one for about $100 from Big Als. This lighting would be enough for plants such as crypts, anubias, and java ferns.