To my understanding the ballast and MH bulbs are wired one to one. There are dual 250's or 400's but they are specifically made for this purpose. I recommend you get one 250W double ended and that should be good for you 65G. Keep your light hungry sps under the lights about 4 - 6" and your lps and other corals on the sides. Good Luck. If interested I have an extra DYI 250 single ended MH capacitor and transformer, no reflector or bulb for $100? and lots of SPS, montis, mushrooms, branching hammer, zoos, ......
Simon