Just because you have 2, 200 Amp panels doesn’t mean you have 400 amps.
Correct. 400 amp services requires 600kcm wire. The demand would never exceed that. So why shell out the $? Answer: You don’t. The higher the volts the lesser gauge wire is required.
Ex 220v at 30amps requires 10awg. 120v a 30amps would require 6awg.