lumatek sells a dimmable 600w dual ballast. controls 2 lamps from 1 ballast. you can get it for like $275. run both lamps dimmed to 400w with 400w bulbs. the only thing is though that it has to be wired 240v. that's an option though.
if not then i would still get 2 400w ballasts. a 600w only has a footprint of like 4x4 really when measuring optimal growth footcandles (5,000-10,000fc). a 400w is 3x3 so 2 of them will cover 6x3. when you have two lights side by side the space in the middle is receiving light from both bulbs. so you can keep them a little further apart. more like 3x7 with those 2 400w lights. so 16sq.ft with the 600w or 21sq.ft. with the 2 400w lights.
run both of them on hps. if you can't afford 2 sets of bulbs then just run hps. if you can afford 2 sets then run the mh bulbs for the first 2 weeks of flower. your plants benefit more from mh light during this period. then switch out the bulbs for hps ones for the last few weeks. if not then just run the hps; much better spectrum for an entire flower cycle. i also wouldn't run 1 mh and 1 hps. maybe if you had a 3rd light i would say do 1 mh and rotate your plants around. if anything i would just get some 6500k cfl's and hang them in the canopy for a more even light spectrum.