You are correct - the ballasts/bulb must match or else. I had a 150W HPS Grow light/reflector. I added in another 70W MH light to the housing, so now its 220W. I remoted out both of the ballasts to a metal box that sits on top of the cabinet.

All you are doing is basically extending the lamp leads and physically mounting the ballast elsewhere, but if you are not comfortable with wiring then perhaps its best to not do it. There should be a wiring diagram on the ballast itself if ya decide to give it a try, or lemme know, I'll help the best I can