Can we use a Relationship between 2 contacts, once in either direction?
Doing some research on your find using svn blame and svn log on the old svn repo, this appears to have been implemented as part of https://issues.civicrm.org/jira/browse/CRM-58, and the key comment is

i.e. if there is "Parent" relationship between 2 contacts then you cannot add another relationship "child " between them.

The code for this is also what's stopping adding the second "favourite". So it seems like the checking is too aggressive - makes sense to prevent the parent being a child of their spouse (except in some geographical regions where it might be true (grin)), but not your use-case.