As luck would have it, I'm a Con Ed employee that looks for diversion (neighbor/landlord taking from a tenant). Electricity can, and in most cases is "jumped" by wire from one breaker panel to another. In most cases, this is found in smaller, multi-family homes, but rarely in larger in apartment buildings (from my experience).
Reducing your overall wattage is the key to reducing a high electric bill. I can say, without you even telling me, that the high bill last summer most likely happened during your August bill (rates were approximately 60% higher than usual....sorry:Yikes
. Since then, the rates have gone down a bit, but still high when compared to other utilities in the nation. I'm not saying that there isn't a possibility of someone else diverting your service, but if you're in a larger building, the chances are slim.
2X250w halides would use more electricity than a single 400w halide if they were left on for the same time period. Bills are calculated based on kwhrs (kilowatt hours). One thousand watts used over one hour equals 1 kwhr, therefore, one 400w halide would need a little over 2 hours to add an additional kwhr to your bill.
I hope this helps a little.......
................E.