Using two gives a more accurate guess at your CO2 level. With one you've got to wonder what color is really green. With a 4dKh solution green should be about 30 ppm. But is it blue-green, green-green, or yellow green? Is it 20 ppm or 45 ppm? With two solutions each has a different 'range' for green. If one is green from 14 to 35 and one is green from 25 to 63, then if both are green it's narrowed it down to 25 to 35. Well it makes more sense with the calculator. I use a 2.93 dKH solution and a 5.26 dkh solution to shoot for 30 ppm. Actually with the shades of green as I see them, I'm probably closer to 35 ppm. I stuck one in there with the standard 4 dKH solution just because I had an extra and I figured it would do more good in the tank than on the shelf.
Once I've got all my fish in there I'll probably judge how they respond and end up taking them all out!