Does your drop checker have 4 dKH water in it, made from distilled water with a tiny amount of sodium bicarbonate in it? If it has tank water it just measures the tank water pH, which is a poor way to do that. Anything in the water that affects the pH will make the pH, KH, CO2 table incorrect, usually making it indicate much too high. Even driftwood can slightly lower the pH. A substrate that has a high CEC can also affect pH, by adsorbing the + ions.
One way to get two input points for CO2 is to use a needle wheel pump to chop up the CO2 bubbles very finely, so they mix and get dissolved in the water easily, then have it discharge at two locations in the tank. Another way would be to split the CO2 line so about half of the CO2 goes to a diffuser at each end of the tank. The challenge is to get CO2 rich water to the farthest point in the tank, before the CO2 is lost from the water surface, or used up by the plants. Obviously the longer the tank, the more difficult it is to do that.