Your idea is a good one, in my opinion. I don't think, though, that most of us can truly say we can read the color that indicates 6.6 pH accurate to +/-.2 pH. It is often very difficult to even tell if the color is blue or green when the drop checker is in the tank. I have been estimating that a regular drop checker, that is "green" means you have 20 to 45 ppm of CO2, but I know that range is optimistic for many of us.

So, just brain storming, suppose you have two bulbs, as you propose, so that when one is still clearly more blue than green, and the other is clearly more yellow than green, the midpoint between the two gives 30 ppm. What would the expected error be? I will play with that awhile and edit this if I can find a good estimate.

EDIT: Here is a combined chart of bromothymol blue solution color vs pH:

The problem with this chart, which is a combination of two charts I found on the internet, is that in making the solution, the last step is to adjust the color of the solution, by adding sodium hydroxide, so it is blue, not green. That tells me that various indicator solutions will differ slightly in the pH at which they will be green.

I did some trial calculations, and the net result is that if you take two numbers each with a +/- X% error, and combine them in any way, by adding, subtracting, dividing, multiplying, the result will have an error of +/-2X, twice as big. So, using two drop checker bulbs makes the potential error twice as big, not smaller.

The example I tried was with one solution being 2 dKH, and the other being 10 dKH. I was trying for having one not quite green, still yellowish, and the other not quite green, still bluish, both at 30 ppm of CO2. That gave me a much bigger error range than either of them separately.

I think there is still a way to make this work, but my brain is refusing to concentrate enough to find it yet.