It seemed that for global bandwidth, we are setting the ADSL downstream speed on 1 NIC, and the upstream bandwidth on the other NIC then assigning them classes for QOS to work.
Unless i have understood wrong?
Ok, you have perfectly understood.
If i wanted to guarantee 80kbps on upload bandwidth and 256kbps on the downstream for VOIP for example, would i have to create 2 QOS classes for VOIP. one for the downstream NIC and one for the upstream NIC?
You shoud not create different classes for the same service. You just have to create a class for any type of traffic and assign it to any interface. If you want asymmetric configuration you must set the local parameter of the class by clicking the [Modify Class] button in the interface manager.