How to show 2 instances of the user profile form, displaying different form modes, on the same page?
You don't need to modify core files, you can modify the "user" entity type in a hook. This is how you add a form class for the form operation change_password:

mymodule.module

function mymodule_entity_type_build(array &amp;$entity_types) {
  $entity_types['user']-&gt;setFormClass('change_password', 'Drupal\user\ProfileForm');
}