Dual coils are fine at higher voltages due to the info posted above (they're two higher ohm coils in parallel). You will get less battery run-time per charge due to this, though. Since the total resistance is lower, it will draw more amps. For example the dual coil, or any other 1.5 ohm item, will draw 3.3 amps @ 5v, while a single coil 3 ohm will only draw about 1.7 amps @5v. Since battery capacity is measured in milli-amp hours, theoretically you would get 50% more run time with the single 3 ohm coil vs the 1.5 ohm dual coil.
But just ignore the geek-speak and go ahead and use them. Dual coils vape great at higher voltages and are well worth a little less battery life IMHO

Higher voltage won't hurt the carto at all unless you vape it dry, but that'll kill any carto at any voltage so it's not anything new to the equation ;-)