So, as I understand. You've got App A, which is available at ABC.com and XYZ.com. So 1 app and 2 custom domains.
The SSO module is able to provide 1 metadata file for a SP, but you need two, one for each domain.
I don't think this is a common setup, at least I haven't seen it before. I don't think this can be done out of the box with the default SAML/SSO module. So you might need to customize the module (change the java code perhaps), and the ability to add multiple SP's..
in Mendix 10 there is way to use multiple IDPS we need to mention idp in the URL
<ApplicationURL>/sso/login?_idp_id=<idpvalue >&cont=<your deeplink>