Let's think about two thermal reservoirs. They are internally reversible. This means they don't have entropy generation(Sgen) right? Each reservoirs' entropy change is different; one is minus(-) and the other is plus(+). But total entropy change in the isolated system has plus value(entropy generation). This means isolated system is irreversible. But reservoir's process is reversible. I think Sgen can not be in the reservoirs since these are internally reversible. And then Sgen may be in the surroundings. How can I evaluate Sgen?