Theoretically, two coils will produce twice the amount of vapor and heat of a single coil. Theory and real life results can vary greatly. I think they are a great idea that just didn't work for cartomizers. Notice that only one brand of cartomizers makes dual coils (SMOK or Smoktech); the rest only make single coils. Single coil cartomizers are far more popular than dual coils.
A 2.0 ohm coil is actually two 4.0 ohm coils. A multimeter will read those two coils as one (2.0 ohm). Higher ohm coils will require more power (voltage), so more power is needed from the battery.
I personally prefer single coil cartomizers. I get just as much flavor and vapor production, and I get longer battery life because my batteries only have to power one coil vs two.
I began using dual coils in my DCT cartotanks many months ago. They worked great initially, but after about a half tankful I often would get a burnt taste to my flavors and harsh vapor. I noticed that a lot of cartotank users had switched over to single coils so I gave some a try. I loved them and never turned back.
I get just as much flavor and vapor as I did with dual coils. I get more consistancy. I get longer battery life.
My theory is the top coil in a dual coil carto lies in the upper third of the carto, well above where the wick holes lie in the bottom of the carto tube. As you vape, the juice level falls in the carto. The wick holes will supply enough juice to the bottom coil, but not necessarily enough to the top coil. If the filler around the top coil gets too dry, it can singe or burn the filler resulting in a burnt or harsh vapor.
cutaway of a punched single coil cartomizer (click to enlarge)
This theory applies only to cartomizers, and not clearomizers which use an entirely different type of wicking system.
