"use the same ID twice, regardless of authentication domain" - I can read this issue one of two ways.
@PaulHomes has already covered making your Oracle AuthDomain outbound only.
Another simple method, if you can't create 2 users with the same Oracle ID, is to create an Oracle-specific metadata group, and store the Oracle credentials in the group's metadata profile, and make the two users members of that group. If they don't have their own Oracle AuthDomain credentials, then each will "inherit" the shared Oracle AuthDomain credentials from the Oracle group.
Best of luck - please let us know how you fare.