How to maintain equal discharging on batteries
Given that you mean (as Ignacio Vazquez-Abrams commented) "drain at the same rate", then it's easy. Easiest, of course, is just to short the batteries. If that doesn't appeal to you







simulate this circuit &ndash; Schematic created using CircuitLab

will do the trick. I've specified Schottky diodes just because they dissipate less power than regular power diodes for the same current. Note that you'll have to provide a proper heat sink for them, while you don't need any such thing if you just short the batteries.