1 ppm means for every 1 unit of Chlorine dioxide you have a million parts of water. So if you double the water without doubling the Chlorine Dioxide you will then have 1/2 ppm.
You still have the same dose of Chlorine Dioxide but at half the concentration. So it might be easier on the stomach. What I would do is drink the higher ppm and then chase it down with another glass of water. This way you don't have to drink as much of the nasty drink but you then dilute it in your stomach.