I couldn't heat dual 28ga with 15 watts very well, but it might be doable with 20 - I'd just get as close to 1ohm as possible, and max the wattage.
If I'm building duals for the GF's istick, I use 30ga. It's no fun using thinner wire, but it will heat a lot more quickly.
Or try single coil, that should work out perfectly fine. And would probably be easier to troubleshoot, honestly. I'd go single coil after you're tired of this build, and if you're so inclined, grab 30ga at some point.
Your coil can have space between each wrap, or be a tighter "contact" coil. I prefer contact, because I'm a fiend for symmetry, and couldn't wrap uniform spaces to save my life

(yeah yeah, screws work, but I haven't bothered).
There's lots of variables to play with, just grab another atty to vape while you experiment with different builds on one, and have fun!