Double the power 4.2 volts to 8.4
You think a standard mech is in the advanced user category, a stacked or series mech is even more so. Single 20amp CDR mech, 4.2v/ohms=amps (ohms law)
4.2v/1ohm=4.2amps
4.2v/0.5ohms=8.4amps
4.2v/0.4ohms=10.5amps <----Safest I run with a 20amp CDR battery
4.2v/0.25ohms=16.8amps <----Safest I run with a true 30amp CDR battery
Series Mech, stacked or side by side box (8.4v/ohms=amps again standard Ohm's Law)
8.4v/1ohm=8.4amps
8.4v/0.8ohms=10.5amps <----Safest build on a 20amp CDR build
8.4v/0.5ohms=16.8amps <----True 30amp CDR
batteries, lowest build you would want
8.4v/0.25ohm=33.6amps <---Not even a Sony VTC3 (30amp true CDR), LG HB2, HB4, or HB6 (also true 30amp CDR) can not handle that build
If you are looking to get one to chuck bigger clouds, nuhuh, not happening thinking because it puts out more volts you can go lower than with a single battery mech, both
batteries have to support the same amp limit and load together, unlike a parallel mech mod the load is split between both batteries, series if the amp load 10amps, both batteries have to support that 10amps, where parallel if the load is 15amps both batteries have to supply 7.5amps each.
Stay safe out there.