I like for each of my two coils to be within 0.1 ohm of each other. If they are off more than that, they wont carry the load equally, and you will get reduced vapor output from the higher resistance one.
I try not to worry about it too much, cuz if I did, it would prolly take me hours to set up a dual.

To save my sanity, I just make sure I cut two similar lengths of wire, wrap both the same number of times, and leave cutting the tails until AFTER dry-burn & pinching. Part of me wants to measure the resistance of both coils separately, but I'm pretty successful in telling him, "NO!"