It’s only possible to have a single SAML provider at a time, sorry.
My first recommendation is that you try this all out on a non-production instance (a copy of your production instance so you can test the migration of users)
It might work. However, I can only recommend following the official documentation.
I think the right move here won’t be to map the external identity to the user’s old e-mail, but to make sure you update the external identity of all your users (preferably by some script) before you transition the users to using the new identity provider.
Basically it would mean following this guide:
Migrating from saml to saml, but updating the external identity to be the @monstarlab-im.com e-mail address.
