Like this?
View attachment 20778
I kind of feel the option to the wireway first then to two service CB's seems cleaner. (Although this way eliminates the wireway)
You are not likely to find a breaker with lugs that will directly accept the combination of conductors you have, but otherwise is acceptable. 400 amp device is likely to have either two port lugs that accept maybe 6 or 8 AWG minimum up to maybe 250, or single port lugs that accept one conductor up to 500 or 600 kcmil but also rated for two conductors of the same size up to 250 or so.
