A "1000w" dual arc bulb is NOT 1000w worth of light. Its 600w worth of light in one spectrum, with 400w worth of the other. Also, the globes are MUCH more inefficient than single arcs, so the 600w side may only be 400 true watts anyways...
Get a 600w hps, and a 400w mh, and run 2 cooltubes next...