The trouble with making the bulbs different is that one is on the back (normally) and the other is on the front. Therefore, one side of your corals will have one kind of light and the other side the different one and a minumum of reflection from surrounding area for the other. Will you turn your corals half way thru the day to give them the mix or will they end up one color on one side and another on the other side? And then, the big question: Will they grow odd because the light is different on different sides? Like a shadow that should be above and below, except this shadow will be created by different lights on one side from the other. Just a thought...