Overdriving 2 20w lights

[Yenko]

I want to overdrive one or two of the 20w bulbs in my 32 gallon tank. I'm planning on doubling the power, resulting in 170% the original light output, or 68 watts. The current ballasts are 2 20w ballasts, one for each light. I have two questions:

If I buy a 4X20w-bulb ballast, can I just double the wattage to one bulb and leave the other running at 20w?

Exactly what kind of ballast do I need?