Great advice so far.
Something else to consider when choosing between dual battery series or parallel, is that assuming you use an appropriate resistance coil you end up in the same place. Two batteries contain a pool of watt hours.
Series - voltages are added together, same mAh same amp limit
Parallel - voltage the same, but mAh and CDR added together(ish)
Exotic coils like multi-core claptons that use a lot of metal and therefore end up being very low resistance. These are better suited to a parallel mod where you have a large combined amp pool.
However, the higher the current in an electrical circuit is the greater the losses will be, ie the more voltage drop you see in the mod which increases exponentially with amp draw.
Because series configuration has a higher voltage pool, much higher resistance coils can be used comprised of thinner wire which is intrinsically more efficient and ramps up/down faster. Less current in the circuit will be more efficient. Less current also means less battery voltage sag.
Ultimately assuming you choose your coil resistance sensibly, you can use either parallel or series to achieve the same output wattage and approximately the same run-time from a pair of batteries, though series is more efficient.
Eg.
Series - 0.7 ohms @ 8.4 volts = 12 amps and 100 watts
Parallel - 0.175 ohms @ 4.2 volts = 24 amps and 100 watts