The way I understand it, a 1.5 ohm dual coil, has two heating elements of 3.0 ohms each. But used together in the same electric circuit, the resistance becomes 1.5 ohm. Make sense? I would guess that if one of the coils burns out, the ohm of the remaining coil becomes 1.5.
I'll state at this point that I never cared for low resistance cartos. I prefer a cooler vape.
I'm not versed in electricity. All I know is how things vape. Having tried dual coils for a couple of months when I got my first carto tanks, I noted that I often got some harshness, sometimes some burnt taste in the 2.0 ohm dual coils on my 3.7 volt mods. It wasn't because the cartos were not getting enough juice delivery to them, I'm certain. I was concerned enough that I decided to do some investigating on ECF forums.
Mostly opinions were found, no real facts. Some said dual cartos were developed by Smoktech to allow 3.7v mods to produce more vapor. Then there was another group that believed that for dual coils to fully exhibit their full potential, they needed at least 4.5v fed into them. Contradictory?

I thought so.
There was a surprisingly high number of people who had switched from dual coils to single coils. Improved battery life was the overwhelming reason. Not enough benefit of increased vapor to merit the battery drain. I decided to give single coils a try.
I bought a whole bunch of 2.o ohm single coil Smoktek tank replacement cartos. I changed out all of my dozen or so tanks with them. I've been using them for about 2 months now, and couldn't be happier. Same vapor production, same flavor. Noticable improvement in my batteries' life, and best of all no more burnt flavor or harshness. For my purposes, the single coils are the winners hands down.
Since my experiment and changeover, I've gotten a Provari. I use 3.0 ohm single coil cartos with it and have been perfectly happy with my results.
We all are aware of the quality control issues that China has been having in the last few months. It seems to be affecting all the manufacturers across the board. I'm wondering if the QC of the dual coil cartos, being that they are so popular, has slipped due to Smoktech trying to keep up with increased demand.
My theory: Perhaps my issue with the harshness and burnt flavor in my old 2.o ohm dual coils was merely from having a single coil burning out and continuing to fire the one coil left, effectively becoming a 1.5 ohm coil. Does that make any sense?
Should you be interested, here is where I order mine:
Single Coil XL Tank Cartomizer for the SmokTank XL
I order 2.0 ohm and 3.0 ohm for my 3.7v and Provari, respectively. I order a bunch of 1 hole laser-drilled and a bunch of 2 hole laser-drilled. Juice with the usual 50/50 pg/vg mix get one hole cartos, anything with more than 60% vg gets the 2 hole cartos.
Anyway, that's my story and I'm sticking to it. Until proven otherwise.