The ZMax will be limited as to the resistance of the coil and the amp limits of the processor in the mod.
In short, the ZMax probably won't fire a coil less than 1.2 ohm. It also will not fire over whatever it's processor's amp limit is.
Stacking batteries in the ZMax has argueably little benefit. Yeah, it increases the voltage output from 3.7 volts to 7.4 volts of a single 18650 battery, but to what gain is that in a regulated mod?
You'll get less battery run time if stacking two 18350 batteries when compared to a single 18650. Stacking in series increases voltage, but
does not increase the capacity (mah run time).
Two 18350 800 mah batteries will have approximately 8 hours run time together. Again, mah capacity does not double with stacking in series.
One 18650 2000 mah battery will have approximately 20 hours run time.