Well, since the AHSupply reflectors are excellent, if you have them mounted directly on (or a few inches up) your tank, two might be too much light for non-CO2. Somebody with more experience can chime in here, but looking at the PAR sticky in the lighting forum (the first chart - the second only has "average" PC reflectors), 20 inches with one bulb puts you in medium light range, which will be the uppermost limit without CO2. Assuming you have a few inches of substrate and a few inches of clearance above the tank, that sounds perfect.
The one issue I can think of would be light distribution. One bulb will leave the front and back a bit dim. Is there any way to use two lights but raise them up a foot?