Boy if those are in the same raceway that is pushing it as much as possible and is only acceptable if the calculated load is less than 368 amps.
250 AL rated 230 amps @ 90 C, four current carrying conductors in a raceway.
230 x 2 x .8 = 368 amps.
240.4(B) allows the next standard breaker however the calculated load must stay below 368 amps.
The last 400 amp overhead I did used one set of 600 Kcmil copper.
In my opinion you are not providing a 400 amp service with 500 kcmil copper (380 amps) or two sets of 250 kcmil aluminum (368 amps).
The service can only be rated as high as the lowest rated component.
Of course take this all for what it's worth, I do commercial work so Table 310.15(B)(6) is out of the question.