I know of no reason why you wouldn't have to derate.
315(B)(2)(a) says

Quote
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.

Greg Fretwell