With dual coils, each coil MUST be identical, otherwise they won't work efficiently. Keep an eye on the number of times you wrap, one of your coils may have one more wrap than the other, and end each coil the same way.
Say you're using a coil jig where you wrap the wire around a post, if you start under the post, your end lead needs to end ABOVE the post, and the next coil needs to be wrapped identically.
Try sleeper builds, rip tripper has instructions on doing sleepers on youtube. Anytime I wrap duals, I build a sleeper.
A sleeper is basically two coils made from the same piece of wire, running through the positive post; if built right, everything heats up more evenly than two separate coils. Separate coils ARE easier, but not as efficient.
Dual coils are typically built in parallel. What this does is split the resistance in half...as others have said above, if both of my coils are each at 1ohm, then the total resistance becomes .5ohms. If I built a quad coil, where each coil were 1ohm, the total resistance would become .25ohms