We have a Quad 40A installed in our main panel back in 2000 to supply the EV charger and A/C. But we can't use both at the same time because the breaker overheats. Perhaps the Sunrun design team has concerns along those lines.
IMO, what you described about your charger and AC sounds like a problem.
Putting two Powerwalls through one Quad 30 A doesn't mean you cannot run both Powerwalls at the same time. If you can't run both of your 40 A loads through your Quad 40A something doesn't seem to be working right and actually seems kind of dangerous. You shouldn't be relying on an OCPD to trip and save you from a recurring problem.