According to the definition in text books, the mutual impedance could be calculated from two-port S-parameters. Mutual coupling would be just another representation of the same data, I think.
Determining the mutual impedance as a one-port measurement requires, that the impedance of a single element has been measured in absence of the other before. Also the mutual impedance has to be calculated. Thus I assume a two port measurement as more reliable. In any case, the mutual impedance is a kind of virtual parameter, that can't be directly measured and is strongly affected by a varied antenna port reference plane.