Well, Here it is.
It's not my fault if you hurt yourself!
1) Start with a twin 40wtt/48" light fixture.
2) Open the shell of the fixture, clip all the wires. (If your wire map on ballast is faded you might want to right it down)
3) Now you need an 18" fixture, the ones that use 15wtt, 20wtt,so on.
4) Do the same as above with the 18" as well.
5) Reconnect the ballast to the 18" fixture, remember that it has twin out puts, instead you need to double the outputs to each side.
6) Make sure no wires are touching each other and that you use something to cover exposed wire.
7) If any wires touch not only will it hurt if your apart of it but it will kill the ballast possibly.
8) You just need a bulb that has the spectrum that you want.
By my use I would say that If your bulb has 1 yr of use plants wise then you over drive it and it is 20wtt running on twin 40wtt ballast; That leaves you with 80wtts on a 20wtt bulb. Since 80 divided 20 = 4, We have quadrupled its output and therefore its life is cut down to 1/4 of its 20wtt life.
1yr bulb = 3month bulb rotation on over drive X4