the bulb is probably a T8 not an 18t. ... and most likely a generic aquarium bulb.
If you cant mount too many lights, have you considered an ODNO setup?
it would be a little more costly, and you would need one, dual bulb ballast per bulb, so you would also need to find a place or way to mount them.
You will increase the output of the light depending on how much you overdrive each bulb, but you will also double the heat output per bulb, and reduce each bulbs lifespan by half.
I forget the exact numbers, but google should turn up many results.
If you use a dual bulb ballast for one bulb, you will increase the bulbs output by 1.25, if you use a quad bulb ballast, then you will achieve a gain of 1.75.
Im pretty sure you will never reach double the output, but basically its just giving the bulb more wattage. Instead of the 18W bulb getting 18W, it would recieve 36W, and hence, burn brighter.
this will ONLY work with an electronic ballast.