If you are open to trying other test platforms, you may want to take a look at NetGend. It's easy to have many concurrent sessions and each will use a different Id. See the following example.
function userInit() {
var globalId = 1;
}
function VUSER() {
id = globalId ++;
action(http, "http://www.example.com/?${id}");
}
There are lots of blogs showing how easy it is to script on NetGend platform.