looking for a specific use-case

I'm looking for a use-case, which I think requires:

  *   that is realistic;
  *   that involves (at least) two people, as e.g. in a marriage, a guardianship or otherwise, and some service provider (SP);
  *   where SP has no earlier knowledge of any of these two people (he doesn't know who these people are);
  *   where SP can obtain credentials from only one of these persons (the other is somehow incapable of presenting credentials);
  *   where SP is requested to make a decision (e.g. to provide a service);
  *   where SP needs to authenticate *both* persons in order to make that decision.

Any suggestions?
Rieks
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