Multiple owners for commerce order [or] Allow non-owner to pay for an commerce order
You may want to look at https://www.drupal.org/project/masquerade module for some ideas to see how it does the switching since it allows:


  "...(anyone with enough permissions) to switch users and surf the
  site as that user (no password required). That person can switch back
  to their own user account at any time."


You can then write a custom module.