Two batteries in a series (stacked or wired as such) doubles the voltage while maintaining the amps of one individual battery.
Two batteries in parallel doubles the amps & mah while maintaining the voltage of one individual battery.
I've read the IPV3 is wired in series. The specs on the chip indicated the mod will fire down to 0.1 ohm. Make sure you start with 2 fresh identical batteries and use them together, charge them together, basically keep them married for life.