Second chamber doesn't help pH, IME. If the reactor is well tuned there is no extra co2 to dissolve the media. Even if there is extra co2 from the reactor, the second chamber rarely gets to a pH low enough to dissolve the media. IMO, second chambers work better in theory or on paper than in practice.
I have Korallin and it barely moves my pH when tuned well. Do you close the top so that excess co2 collects? If you let the co2 out by using the top port, then tank pH can lower with the extra co2.
I keep my home very well aired out with co2 less than 500 ppm most of the time and the CaRx for a very calcium/carbonate hungry tank from a Korallin 10000 is like 8.31 instead of 8.34, or something like that.