@endorphinThanks for sleuthing that, that's really useful.
I think there's still ambiguity in it, though; the FAQ in its entirety seems to be written from the point of view that you are only managing a single WhatsApp account. It explicitly acknowledged the possibility that you might have multiple Google accounts, but it's completely silent on the scenario of having multiple WhatsApp accounts.
Plus, the fact that it says restore from backup might fail if, and I quote "You aren't using the same phone number that was used to create the backup" - seems to imply that your phone number (and therefore also your WhatsApp account identifier) is stored with your chat backup.
Still, who knows? On the plus side, that same page gives some incredibly useful information about managing local backups. So it looks like it's possible to transfer local backups into manually created separate folders on Google Drive (or even Onedrive or Dropbox for that matter). Obviously I don't expect backups moved to different locations to restore automatically - but the point is, with this information, you could set up a test to see if you can automatically restore two different chat histories from two different WhatsApp accounts on the same Google Drive account - and you can manually recover the histories for each WhatsApp account if it turns out that it doesn't work.
Maybe I'll try that at some point, thanks again.