This does make sense, if your cells were designed to run on the lower voltage. Drawback in one more BIG thing to find a place to install. But, it wouldn't be hard to build a simple circuit that would connect the HHO battery to the vehicle electrical system if the HHO voltage dropped below 11 volts, then disconnect once the voltage rose to a a preset level -- kind of a back-up charge for long trips.
1991 Plymouth Acclaim 3L V6.
1 dry cells with nineteen 6"x8" 316L ss plates, driven by constant current PWM set at 35 amps (13.3V at PWM). 28% KOH electrolyte. Total measured output 2.5 lpm. Mileage went from 18 to 26 mpg, all city driving (44% increase). EFIE set at .370 and I still need to play with ignition timing.