what I have seen is metal halide with integrated halogen bulbs, ie the circuitry to switch between lamps is integrated into the ballast, not remotely switched. The fixtures were on an emergency supply, and the halogen bulb would come until the halide struck or if the halide failed.
I will see if I can find a link, it is not quite what you were looking for, but the fixture was designed with two bulbs.
Jay8: What you describe is a 'quartz restrike' option, usually on hi-bay HID fixtures, to provide illumination while the HID lamp is re-striking from a power interrupt.
Re-reading the OP, and his second comments, it sounds more like he's looking for some type of 2-level lighting that uses two different light sources. I've seen & installed 2-level lighting, but it was a single light source (MH).
I am not sure but I have a feeling that there is a company do make specal set up this company called " Bodine " they do have something for HID's and here the link .,,
