When I maximize any application on one of my monitors it extends approximately 1 pixel to the second monitor.
Not too distracting except when I use Toggle Present Windows (Ctrl + F9) to see all my open applications, and on which monitor they are open, it shows the maximized applications as being open on both monitors.
I have two almost identical monitors. A Dell S2721DGF (1440p 165Hz) and a Dell S2719DGF (1440p 155Hz OC). Both of these are plugged in via DisplayPort. Not sure how to go about troubleshooting this one.