Each receptacle is rated for 20A, and the whole device is rated for 20A.
So you can draw 20A from the top receptacle.
You can draw 20A from the bottom receptacle.
Or any combination less than or equal to 20A.
But you cannot draw more than 20A on any part of the device.
Looking at the guts of a receptacle, might make it a bit more clear.
When the tabs between the terminals are in place, a total of 20 amperes can flow through the duplex receptacle.
However, if you remove the tabs, each set of contacts is still capable of carrying 20 amperes. Which means if the receptacle is supplied by two separate 20 ampere branch circuits, you can power 20 amperes worth of load from each set of contacts.