fish i believe u r half right.
total resistance is still 1.5ohm no doubt its dual coil.
So like u stated, its actually 3ohms per coil inside. But output resistance is still 1.5ohm.
Then thats where the mah comes in. using small batts, u will fry them reason being their output power is not enuff to power "2x3ohm coil" at the same time. The coil will quickly drain the batt and its not good.
(n thats y u see i stated 650mah n above) (edit: i think its safer for 900mah n abv)
Therefore when using a dcc, just have a high output batt (high drain) but voltage remains at 3.7-3.9v
(thereabout. find ur own sweetspot lol).
Correct me too if i'm wrong.
p.s. its parellel. not series =X