Two meters? No problem!
The good news is that the problem you're trying to solve has already been solved for you, as multi-pack metering systems for duplexes and small "plex" apartment buildings are widely available these days. In particular, assuming that your ADU is a detached structure and that your local Code authority is alright with existing grounds (i.e. some light fixture out in the middle of the yard) lighting staying on the existing house's panel, you can use any EUSERC-compliant two-meter pack that supports the loads you are working with.
That said, that tankless heater is really a drag
However, that 20kW tankless water heater you're proposing for the ADU is a bad plan, given that it's basically doubling your load calculation for a very marginal gain in usable space. Instead, I'd recommend using a "low boy" style tank water heater tucked into a cabinet space to at least be able to have a usable counter over it; there are also a few options out there that build the water heater into what's basically an appliance-shaped package, but those aren't as widely available as the more conventional "low boy" tanks are.
Putting this all together...
Once you jettison the tankless heater, then we can talk about service sizing, as you shouldn't have any difficulty getting the ADU onto a 100A service once the tankless goes bye-bye, and that means you can use a 2x125A meter-pack for this job. I'd recommend a Siemens WEP2211, but that's because it's inexpensive and not hard to find; most other manufacturers of multi-pack equipment should have something that fits the bill as well.
The other good news is that since PG&E considers a duplex to fall under single-family residential service rules other than the separate metering, you don't need to worry about having to deal with more than the standard 10,000A of fault current any off-the-shelf setup can handle. (If you did have to worry about that issue, you'd need to order special main breakers with a higher interrupting rating than the regular ones.)
(Image is from A.O. Smith's website, but that's not an endorsement of them in particular -- plenty of other manufacturers make lowboy electric tanks!)