Hi Evan,
The way to allow users to select between multiple authentication sources is to use a "discovery page" in the SSO settings under Authentication.
Each different authentication provider that you set up in Canvas gets its own URL assigned So, set up both SAML and CAS providers, and make note of the URLs. They will be something like https://<canvas server>/login/saml/2 , etc.
Then, create an HTML page on an external server that includes links to each of the different Canvas authentication provider URLs. Instead of using the order of the authentication providers in the Canvas list, you can put the URL of the "login page" under SSO Settings > Discovery URL, and instead of being automatically directed to the first authentication provider in the list, when they connect to the main Canvas URL, they will be directed to the "discovery"/Canvas login section page that you have set up.
Mike
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