when they say it accepts metal hallide and hps that means its a convertable ballast which it can accept "EITHER" one bulb or the other in order to run 2 bulbs at the same time at least the only way i see it is that the ballast would have to be double the wattage of each bulb. but any ways to answer your question it only accepts one bulb at a time. Also my friend has a digital ballast and it is near silent.