Scenario
You want to be able to programmatically set a Mult-User column of an SPListItem. The trick is to build a string that represents the users in the following format
SPUser user1 = {some SPUser};
SPUser user2 = {another SPUser};
string multiUser = user1.ID + ";#" + user1.Name + ";#" + user2.ID + ";#" + user2.Name
So your string should look like this
string multiuser = "1;#Jason Noble;#2;#Administrator";
Then you can use the standard syntax for setting the SPListItem's column…For example
myListItem["AMultiUserColumn"] = multiUser;
Hope this helps…