Unless I miss something it seems impossible to have access to all your vaults on 2 different onedrives (E.g. Onedrive Personal and Onedrive Entreprise) from the IOS app. It is only one or the other.
Yes that’s correct. You can connect the iOS app to one OneDrive (that includes OneDrive for business), one Dropbox, one GoogleDrive, one iCloud Drive or various WebDAV connections.