i did some tests on a friends tank. (built him a second chamber for 20$ in acrylic) and his raised from 6.5 to 6.8-6.9
i think there is variance between peoples setup because some people run different flow rates through them, and also you have to think if the second chamber is a tall or short one. (taller will mean more water hits more media before exiting while a short fat one might hold the same amount of media, but doesnt hit all the media)
The aquarium engineering one listed above is made by bill wann do a google search and check out his tank and system. its cool
but his is a slight copy of the schruran reactor. (as far as sucking effluent water from 1-2" from the lid where some CO2 can puddle. and it also has a 1/4" line going from the lid to the pump to re-circulate any CO2 back into the reactor.)
on my MRC, i stuck a 1" piece of RO tubing into the lid fitting so my reactor pulled effluent from down in the reactor.
my setup might not have needed the second chamber since i only ran my PH at 6.8, but it doesnt hurt, doesnt use any extra electricity.