You happen to be incorrect; you can already buy a splitter to run two 600W HPS lamps from one thouie ballast. However, unless that ballast can push 1200W, you're only going to be driving those lamps at 500W ea, not ideal.
Moving on; the most efficient HPS lamps are 600W, followed closely by the thouie. Smaller ones just don't have the lumens per watt punch, nor the spectrum to keep up with their big brothers.
While you can in theory run, say, 4 x 250W HPS lamps in series on a single thouie ballast, there are more disadvantages than upsides. For instance, if one of your bulbs fails, the others quit, too. There's extra expense, and of course you don't get the same performance for your kilowatts as with the thouie.