I think you got it a bit wrong .
For every MH you need a MH ballast and for very HPS you need a HPS ballast
OR !
You can look and buy a multiple-balast, for boht MH and HPS , I have about 4 250 W multi balast fixtures working just fine, and I just switch the bulbs when I need to .
BUT, for you to use a MH bulb on your HPS fixture, you must first consult the HPS balast and see if it does suport both, in most cases if you didn;t buy a multi use ballast, it;s not . .
hope that helps .
peace !