Interchange values in "people and group" fields

Hi,

I have list with two people and group fields. How I can interchange values between these "people and group" columns effectively without loosing information?

String ValueA = item["Field1"];

item["Field1"] = item["Field2"];

item["Field2"] =  ValueA;

Is not working for. Should I use SPFieldUserValue or SPFieldUserValueCollection? Could anyone provide me better logic with example

Thanks in advance

HR