Hi Marybeth, thanks for the question.
Passowordless login could be enabled on any number of domains. However, auth sessions are not shared across the domains. The access to the backend resource server through the second domain will be possible once a session is established to it.
If using Passkeys, different passkeys will be stored for the different domains. Support for the ability to use a single passkey for authentication over all domains through passkey related origins is under evaluation.