Two 24"s or one 48" for 55g???

one light wont be enough....its between 36-40 watts for a 48" Flourescent and thats less than 1 wpg...if you need something for the time being and dont mind it being ghetto...go to Home depot, they sell the industrial light fixtures for outside....you can get a 48" one that holds 2 bulbs for 7 dollars.....go get two of those fixtures and 3 40w 6700k bulbs from either the LFS or Home depot (home depot usually being cheaper) and then either 1 10,000k bulb or one actinct bulb to make the color not so yellow. Hang the fixture from the ceiling for the time being above the tank...voila!! cheap lighting...and now whenever you get the chance you can build a hood out of wood (cheap) and then fit the fixture you will allready have into the hood!! THEN you will have a nice setup!! So total that will be about 160watts so about 3wpg not too bad. And it will cost you 14 bucks for the fixtures and about 5 bucks a bulb. So 40 bucks....all in all cost effective.

Thats what I'm doing!!
Search homedepot.com for shop lights

one can work

I am growing just narrow leaf java fern on a large branchy piece of dirftwood in a 58 gallon tank with one 30 watt bulb. So, that is 0.5 watts per gallon. But, most of the fern is up high, some is low but none is shaded by any other so all get some good light.

If your aim is to be verrrry simple, it can be done with one light.

But, trust me, you really ought to try to get better lighting. This is my 5th active planted tank, and if I only had one, I'd have to have more plants than that!