One 1000 watt ballast is equal to 8 amps (normal metal halide or HPS,not sure about the new fancy electronic ones), so therefore running two 1000 watters on one outlet requirs a good 20 amp wiring configuration.
Keep in mind the typical home has one 15 amp drop to each room (not even that usually), that in turn is daisy chained to the other outlets in the room (or rooms). If thats all you have, then plan on adding a new curcuit breaker to the panel, and running another drop to the room, or two to be even happier
I had that problem, and lucky for me I don't have a land lord, so I just added two new lines to my room. Id trip fuses all day if I hadn't done that.