If your reactor is tuned well, then a second chamber does nothing since there is no excess co2.
I would figure out if you want to let a pH controller run your reactor or if you want to learn to tune it yourself. A reactor with a pH controller will never be all that efficient and they will not work at full-tilt, but some people just don't care and do not want to bother to learn how.
If you want an efficient setup, then buy a reactor that traps co2 in the top so that you can tell when it is out of tune. Korallin and some others do this. Nearly all reverse flow models can be modified to do so. If you don't care then get a dual chamber GEO setup since you will likely need the second chamber to help with your wasted co2.
In the end, the air in your home will still have a much more significant impact on pH than any reactor.