yeah, that's what I did. only have one ballast right now with an HPS and an MH conversion bulb. Only problem is that when you're veggin, that's all you can do because of the one ballast.
yeah if u only have a hps bulb and a mh conversion bulb then you cant veg and flower at the same time and thats a bit of a pain in the ass but its better than cfl's by 75%.
instead of buying another pre-assembled ballast get a ballast kit from htgsupply.com and put it together. i think you can get the shit to make a 400watt hps magnetic ballast for $60-$75. then you can veg and flower @ the same time.