Multiple SSO providers per user

This can only be done by the domain admin, and you can achieve it with the help of API: Update User Information API
Example:

{
   "accountSetting": {
     "firstName": "Jane",
     "lastName": "Doe",
     "ssoProvider": "sso_provider_1, sso_provider_2",
     ...
     "language": "en-US"
   }
}