The supercharger is boosting or compressing the air that enters it's inlet, and is generally fixed by pulley size, unit capability, and rpm.
If the turbos were not hooked to the sc, than atmospheric air pressure (around 14.6 psi, depending on the altitude) would be increased to whatever the sc's capability is, say 11 psi more. If the turbos were attached and producing 10 lbs of boost, then the total boost that the engine would see would be around 21 psi + atmospheric pressure. The boost pressure that is measured generally does not include atmospheric pressure.
In Torries setup, as well as most turbo vehicles, the waste gate valve opens and dumps exhaust pressure before it gets to the turbo, to control the speed of the turbo, and its ability to compress air.