It more depends on your e-cig. Dual coils split the wattage between coils. It takes a lot of amps to do this at high voltage and low resistance so YMMV depending on your setup.
Basically, the best way to figure out what you can do is to use the Ohm's Law Calculator. Type in your voltage and the resistance and hit calculate. If it's between 6-8 watts divided by 2 then you're good to try it. If it's not, then forget about it. You can play around with the numbers and see what may or may not work for you. I could tell you more but I'd need to know what your vaping on for an e-cig and what resistance you are accustomed to.
Ohm's Law Calculator
BTW: Power = watts. Watts = Power
Thus, as Elmattias said, it may be more beneficial to rely on a single coil. Furthermore, more vapor isn't always the answer. If you don't believe that do a double vape (two batteries, two atomizers at the same time). I bet you hack your brains out. I know I did. BETTER vapor is the answer and that requires the right mix of equipment, voltage, resistance (the combo of which will give you your wattage), and juice. Long story short, it's best to have a basic understanding of how electricity affects vaping. If you'd like a primer I explain that in great detail as well as a great many other things in The Vapor's Handbook. You can find the link to it in my signature and I invite you to go there and learn all you can. After all, I wrote it for all vaperkind.