Mu and Mu_prime in S parameter analysis

Hello,

I have a question about the mu and mu_prime parameters in the S parameter Direct Plot window. I know what they are but here it is written for both mu and mu_prime that them being greater than 1 is the sole necessary as well as sufficient condition for unconditional stability of the 2-port network. How can this be independently true for both mu and mu_prime? 

Thank you