i would use 2 hps instead. but 1 of each will only make things better. plants actually benefit from both the red and blue spectrum though. but unless i was growing an exotic sativa, i would use 2 hps instead because plants generally like the red spectrum better during flowering which is provided by the hps. a guy i know was growing with one of those dual bulbs, its like a hps and mh combo bulb. he eventually just switched to only hps because he said it gave him better results. however, i was reading a newer skunk magazine the other day, and they say certain exotic sativas(i think these types sativas were more for outdoor grows) need both spectrum to achieve best results. so if you have one of those, then i would go with both mh and hps.