Where "disable concurrent logins" is checked and you're using vouchers, you'll see something like this in your portal auth log when connecting a second machine.
Jun 8 21:25:08 logportalauth[72742]: Zone: zone1 - CONCURRENT LOGIN - TERMINATING OLD SESSION: BvCaK7yXvRa3, 00:50:56:a7:5a:58, 192.168.155.101
Jun 8 21:25:08 logportalauth[72742]: Zone: zone1 - Voucher login good for 5 min.: BvCaK7yXvRa3, 00:0c:29:cc:8c:a3, 192.168.155.102
That removes the original session and adds a new one for the new MAC. It works, I just ran through testing of it again on 2.2.2.