What are these "patches" on an Airbus A321 wing?
I am not familiar with that particular aircraft but I would hazard to guess it is not a patch.  I say this as I have never... in 20 years of aircraft maintenance... seen a patch that incorporated screws.  If this were a patch for a crack in the sheet metal it would be rivets all the way around.

AC 43-13 standard practices is the general reference for skin structural repair although manufacturers will publish their own Structural Repair Manual with their own preferred methods.  And certainly I have seen design Engineers come up with odd things to deal with AD's etc.  But I have never seen screws used as a method for installing a patch.

So... I would say it is likely a designed in strengthening piece. It is also a single piece if you look at it carefully it contains a Joggle which makes it look like two separate pieces.  A Joggle being a step in the metal to account for two different surface heights.