Don’t forget 310.4(D) that refers you to 310.15(B)(2)(a)
More Than Three Current-Carrying Conductors in a Raceway or Cable. If you have 2-1/0 Cu. conductors @ 75?C, you have 150 amperes permitted in each conductor. However, if you have three sets of parallel conductors in the same raceway, you have to take a reduction. You would have:
150 X 2 = 300 amperes per ?.
Then, since you have 6 current carrying conductors in the same raceway, you would have:
300 X .8 = 240 amperes per ?.
It may be well to consider using 90?C rated conductors so you start with 170 amperes per conductor and that will leave you with 272 amperes per ?. Not quite 300 amperes but 32 amperes closer.