Can you see any obvious challenges to requiring 2+ users to confirm authentication requests using an association such as a friendship type of relationship.
A practical application would be an Enterprise SAAS application that requires another user in the same company to authorise his login request within a short period of time? It would then be up the the users company to decide how many authorisation confirmations they require. (Usability vs Security)
And edge case would be if a company only had a single user, they could simply register for 2 accounts, ideally using 2 seperate machines on different networks (LTE & ADSL).