The dual chamber reactors typically have CO2 injected water circulating through only the first tube; the output of that tube (AKA effluent, as Texreefer explained perfectly)then runs passively through a "static" tube of calcium carbonate media, with no circulation pump or CO2 injection. The extra contact with the media tends to raise the ph of the effluent before it exits that tube into your tank.

But, if you're also going to run a KW reactor, you don't have to worry about it. As I said in my earlier post, any carbonic acid in the reactor effluent will quickly react with the KW in the tank; in fact KW needs to react with CO2 (carbonic acid) in tank water to produce carbonate ions from the hydroxide. So, I would suggest if you're running both a calcium reactor and KW, just use a single chamber calcium reactor.