If in two seperate conduits 300 kcmil is good for 285 amps at 75c. see 240.4 (b) (800 amps or less next higest breaker size) .
240.6 300 next breaker size is 600.
285 plus 285 = 570
if in one single conduit see table 310.15(B)(2)(a)
4-6 80%
570x80%=456 next higest per 240.6= 500amp.
Same Conduit would require 400 Kcmil
335x2= 670 x 80%= 536 . 240.6 next higest 600
This is with 3 wires in conduit single phase. Or
6 wires in conduit single phase.