We have a multitenant application where each tenant can have it's own SSO setup (using the SAML module from the appstore). Each tenant has it's own URL which will redirect the user to the external login page of that tenant. After the login the user is redirected to our app again. We do have our own Mendix login page. On this page is a button for SSO login. The user has to provide an mail adres so we can then determine which SSO setup to use for redirection.
Hope this helps.
So basically the flow would be something like this :
Mendix login page → enter your email → page redirects to SSO url based on the email
Is this how you are handling it ?
Also make sure the entityDescriptor has a unique value in each idp-metadata file. Each IdP-condiguration should have a unique IdP-metdata file.