I like the idea, but after looking at it, I have a way to improve on it.
Use the top res to top-off the bottom res. Use the bottom res to water the plant with. Run the
pump to the drip lines, and as the bottom res empties the top res via the float will keep it topped-off. This way the pH/ppm in the top res will always be right and should keep the pH/ppm in the bottom res correct.