Dual coils are something that looked great on paper a few years ago but didnt yield the benefits that people were expecting especially with cartomizers.I can imagine the same holds true for RBA's.
Id stick to the single coil IMO.The more parts you have the more apt something is to break or go wrong.
I would tend to agree with zapped, to an extent. If someone is a vapor hound and wants the most dense thick clouds of vapor possible, dual coils is one way of going about it. Using 28ga wire, you can only get down to about .6 or so before juice starts to burn due to overheating. Using dual .7 ohm coils (.35ohm overall) will allow you to increase performance even higher than the .6ohm (roughly) ceiling that 28ga wire has.
Going up to 26ga wire you could match the same performance, but without the hassle of dual coils. If you aren't looking for the most extreme, over the top vape, dual coils are just a waste of time. And if you are looking for the most extreme, over the top vape, there are easier ways of doing it.
Not trying to be discouraging here, they sure are fun to build and if you love tinkering go ahead and try it out. But there really isn't any performance advantage that can't also be had via other means.
Now dual coil cartos are a bit different. They are totally useless unless you have a PV that can pump a lot of voltage into them. A 1.5 DC at 5.2v or so will outperform any other carto and will approach a sub-ohm RBA in terms of vapor and TH (although NOT flavor) Nobody builds a single coil carto that can handle anywhere near that kind of power, and you can't rebuild cartos, so DCs at high voltage are the only option if someone wants to stick with cartos but get massive vapor. Stacking 18350s in your VV/VW mod of choice will likely be needed, as with a single battery they have trouble boosting the voltage that much with such low rez stuff.
You say you have a Cobra, they can be easily converted to dual-coil/dual-wick by drilling a single hole opposite of the original wick hole. The center post stays where it is (obviously) you use the original wick hold for your first coil (with the standard negative screw) and then you use the fill port screw as the second coil's negative terminal.
