one to one chat

You could use a remote SharedObject. In that case you wouldn't need to pass in the user to connect to in the SSAS at all -- just append an object to the remote SO that identifies the intended recipient. Then onSync check if you are the intended recipient, and if not, discard(do nothing), otherwise show the message.

However, it would probably be better to do just the opposite, use all SSAS and don't use a SharedObject at all. To do that, you could initialize an object/array onAppStart, like this:

application.onAppStart = function(){
this.userList = {};
}

Then put new logged in users there:
application.onConnect = function(newClient,username){
if(!this.userList[username])this.userList[username] = newClient;
}

Then create a function to send private messages:
Client.prototype.sendMessage = function(msg,to){
application.userList[to].receiveMessage(msg);
}

Then on the client .swf, add a function on your NetConnection called "receiveMessage" that takes the message as a string and do what you want with it. Done!