The chargers have step up transformers to reach high voltages. Good luck bumping em up to 3 phase. As far as service voltages go, a 400a single phase service is a rare animal in a residential building. The square footage would have to be 10000+.
More likely is 2 200a 40 circuit panels. In areas that use natgas for heating and cooking, a 400a panel is ridiculous.
A 100a sub branched off a 200a main doesn’t mean you have 300a.
Clown.
Just because you have 2, 200 Amp panels doesn’t mean you have 400 amps.