If you regulate the BPS perfectly it never switches off.. This you can do with using it n conjuction with a drop checker and a Ph profile.. Once you know where you're at you can set the controller 0.1 or 0.2 pH higher than the desired target. But it will never reach this target since it's set higher and thus never switch off. It'll ony switch off if the BPS exceedes the target value. Or if the controler needs a recalibration, tho personaly in the 2 years i used it it never happened, did a weekly calibration anyway at each water change.
This way the controler gives you a permanent pH reading and works as a fail safe in case of a tank dump it switches off and not as a permanent controller to regulate co2 administer. And it shouldn't be used for that, it is not a easy going laid back automation device to forget about CO². It should be installed with a timer switch on the solenoid as a fail safe and a permanent on the fly pH reading.
A tank dump also can be prevented with buying a more expensive dual stage regulator.. Tho not so log ago a UKAPS member still sufered a tank dump with an alleged duall stage, the vendor was awfully sory and appologized rectified the issue, but still all his fish were gassed and dead. That's quite a shitty situation trusting on technical equipment and when find out it fails it;s to late.
Than you can ask yourself the question are you beter off with a cheaper single stage regulator and a controller? Both added likely are equaly expensive as a dual stage regulator.
That's a question everybody has to answer personaly for themselfs regarding their budget and believes. In the 2 years an 10 litre CO² i used with it, i still monitored it closely every day. And it actualy served me pretty well. I can not second all the horror stories giong around about pH controllers.. I honestly have no idea why, with what and how the people claiming it ran into the problems.. They probably used it differently than i did. And blame the device.
Using 2 solenoids on one controller? I can't see why this would do any good.. If one controller controlles both than they both switch off at the same point.
If you have one solenoid controlled and the other not than i stil can not see the point of it.. No idea where you are going with this..