I'm using two, bro. I used a backup Pinpoint pH monitor to dial in my readings from one reactor vs. two, but am keeping two pH probes actively on the Apex. I had to get a PM1 module because I have the Lite version of the Apex, not that it matters much anymore.
I really don't see much of a difference at all in running two chambers vs. one, to be honest. I love the numbers I'm getting, but I almost think that a one reactor chamber would've been more than suffice.
Tonight, I only tested Alk (9.1dKH) and Ca (479ppm). I did some tuning to try and keep it around there. The pH is at 8.04 for the whole tank right now. It's much easier to tune the Ca reactor with the Hanna checkers I have, too.
Any way, thanks zibba, rev and P for the compliments, as always. :) I may have to work on some splice work downtown tomorrow, so I may not get to fiddle with the system too much. :( I still owe a bunch of updated shots on the corals, too. I'm getting some decent, yet scary, growth out of some of them. :D