I have separate business and family Proton accounts, and I really need to have synced folders for both of them. But Proton Drive for macOS only supports one account at a time.

This really discourages widespread adoption, btw. If I work for, e.g. a consulting company that itself uses Proton, and we have clients that also use Proton, and those clients want to hire us to work in their files through Drive but we're already syncing our own corporate Drive, how would that work? Proton Drive really needs to support syncing with an arbitrary number of accounts/drives.