In reality it would be a better aproach to use the CaRx as a baseline and dose 1-2-3 part on top of that controlled by the KHD.
So for example if your tank is using 2 dKh /day alk, then set the CaRx to let's say 1.5dkH/day alk replenishment, and do the 0.5 dKh by dosing. That way the KHD can adjust that 0.5 dKh up/down easily.
With controling the CaRx with a the KHD there are two problems:
1, You'll have to set the target CO2 -> PH below the actual desired value-range (let's say for 2.5 dKh/day instead of 2), and the solenoid will be clicking all day to keep the ph above.
2, The reactor is slow to react to any changes really, so I can imagine it is very hard to pinpoint a certain daily dKh value this way. (For example when you shut down the CO2 feed to a CaRx, it still pushes Ca/Alk into your tank for a couple hours as the ph equalizes).