Even newer developments:

Fixed the tabbing issue by rebooting. I just tried having both instances run in windowed mode but with only ONE maximized; if I minimize the un-max'd instance onto the taskbar, the other window is artifact-free; however, if I pull up the non-maximized windowed-mode instance and have it running on top of the maximized instance, that instance will then have the artifacts if it is on top of the other window.

Gibberish-free version:

If a window is on top with a non-minimized (i.e.: NOT minimized onto the taskbar) instance running directly behind it, it get tearing and artifacts.