Yes, I'm running 2 x 1000w ballasts split into 4 x 600w lights, plus I have another pair of 600w ballast/lights in my grow. I've run this setup for a few years now without problems, although one of the ballasts burned out after close to 2 yrs and don't know if the Sunpulse splitter was to blame (the other ballast is still going strong but then it is a better quality rig).
I think it's a good solution - I was faced with the same decision to either get more 1K rigs or step down to 600w which a lot of growers seem to prefer, plus it does cut down on heat from another ballast and saves some energy, although IMO I think the split lamps aren't really putting out 600w, but then I've never measured how one bulb performs in both a regular 600w ballast compared to the split 1k. It might be worth doing a little experiment with my light meter ;?D