This is the way! I tried to get Tesla to do this and they wouldn’t. I did the load calc and can get by with just 200 amps for house. Tesla can back and suggested I replace one 40 circuit panel with 80 circuit panel. I thought about it and accepted that solution just so we could move forward. It didn’t have to go that way, but I wanted powerwalls.
To OP, feeders don’t have to be individually current protected. But do to emergency disconnect rules you might requir a breaker as a disconnecting means….
You jurisdiction should take a years worth of data in place of a loads calc, so pull you maximum instantaneous power from your panel. Can you down load each panels data, put in spreadsheet and add both panels together and find the peak…
I put my chargers on the non-backup panel, but I have a 50A/240V receptacle in garage connected to the backed up loads panel and have the Tesla mobile cable I can use to charge from solar during a grid down event if needed.