Since we are really talking about WebAuthn and not just the iOS implementation, You probably could use alternative hardware (maybe android based) that supports the multiple-user biometric use-case. From the server/web-app side it still would just be WebAuthn.

Looks like you use a separate device like this: https://www.ftsafe.com/Products/FIDO/Bio

That supports fido2, hooked to any tablet to make it work.