The primary auth method for a scope “auto-vivifies”, creating a Boundary user object for the corresponding account automatically so the user can log in, but other auth methods do not. If you use separate orgs and have each one have one of the auth methods as primary, though, you can have both methods auto-vivify within their separate orgs.
The other thing you can do is create users for the corresponding accounts on non-primary auth methods either automatically by using Terraform or scripting the Boundary CLI, or manually using the Boundary admin GUI.