To reach k set consensus (k=2)
Quoted from Chapter 16 of the book:
Distributed Computing: Fundamentals, Simulations, and Advanced Topics (2004):


  (Page 345) We now show that there is no algorithm for solving $k$-set consensus in the presence of $f \ge k$ failures.


and


  (Page 351) Theorem 16.7 There is no wait-free algorithm for solving $2$-set consensus problem in an asynchronous shared memory system with three processors.


The proof is quite complicated. See also the paper: The Asynchronous Computability Theorem for $t$-Resilient Tasks.