There really isn't a 2x18w or 4x36w ballast. What you'll generally find at Home Depot is a 4-F32T8 ballast that is a load-sensing electronic ballast to run 4 bulbs. If you hook up a F15T8, F32T8 or even a F40T12, the ballast will sense the load and deliver the "proper" wattage to each ballast ouput.
So if you hook up a 18 watt fluorescent to this ballast with only one ouput, you won't be overdriving it to 32 watts (which is the maximum ouput each lead can deliver). I had this same misconception until I measured current draw in various configurations and determined the ballast was load-sensing.
Anyway, the setup I'd use for your two 18watt tubes in series is to run two outputs to them (2x ODNO coaxes about 1 1/2 times more bulb output, 4x ODNO coaxes about 2 times bulb output - illustrating what DLeDeaux meant by diminishing returns). So your two 18w tubes would deliver 54 watts instead of just 36w.
This is the wiring configuration you'd use (except use 18w tubes instead of F20T12's):
This diagram also shows all four ballast outputs being used (4x ODNO four approx double the bulbs normal rated lumens output). Try using only two of the outputs first. If you're not satisfied with the output, you could hook up the other two outputs, but add a fan since the bulbs will get pretty hot and won't last as long.
Even better would be to run two pairs of 18 watt bulbs in series, each being run 2x ODNO, for the most efficient output. Four 18 watt bulbs x 1.5x output = 108watts! *Tim Allen hua hua grunt*