I know of no reason why you wouldn't have to derate.
Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.
I get 416a per pair for THHN/THWN-2
260a for a 4/0 @ 90c x .8 (derated for 6 in the pipe) gets 208 x 2.
That does not take into account the voltage drop which might actually be the limiting factor.