Reconfigure Thunderbolt DisplayPort Monitor Ordering
System Prefs > Displays > Arrangement.

Drag one monitor over the other to the opposite side, then move the Menu bar to whichever monitor you prefer.

The actual screen is given a large red outline as well as the representation in the control panel as you drag, so you're certain which one is which.

BTW, that tenacious holding onto which display is which is a whole lot better than it used to be, where it would randomly decide 2 identical screens were the opposite way round on occasions - back in 10.6 &amp; before.