Ignoring the fact that you can't parallel small conductors for a moment, you're assigning half of the current to each wire. Seems that each wire emits half the heat, and there are two of them. That works out to be a wash. Plus, you now have one more CCC to consider.
I agree.
So with that being said, the de-rating issue shouldn't even come into play even if all the conductors were energized simultaneously.
JAP>
If you have other circuits in same raceway, your total current carrying conductor count still goes up and therefore your adjustment factor may change. The "travelers" would only be carrying half the current they otherwise would carry if there were a single conductor - they won't need to increase in size because of a different adjustment factor, as they are carrying less to begin with. Those other circuits in the same raceway may require increased conductor sizes though.
Remember for fixed lighting and a 15 amp branch circuit - you don't have to be able to carry 15 amps on the conductor - you only need to carry the actual connected load. General use 15 amp receptacle circuit you need to figure you might be loaded to 15 amps and have to make adjustments based on 15 amp load.
More direct response to OP - three way "travelers" only carry current on one of them at a time, you only need to count one conductor as current carrying for ampacity adjustment reasons. Simple two wire "switch loop"- both conductors carry same current at same time. Throw in a neutral as is now required for most lighting applications - technically that neutral is a current carrying conductor if not arranged to carry unbalanced current. If you only have one phase involved in the "switch loop", then the neutral is definitely a current carrying conductor - though it typically will only be carrying load in the mA ranges at most.:blink:
