Two thunderbolt ports on my mid-2015 MacBook Pro how do I configure one monitor to be in mirroring mode and the other in extended desktop mode?
It is not possible to configure such a setup with the standard macOS GUI.

However, I think you might have understood something, and you do not really need such as setup.

If you want to be able to have 2 separate, external, 4k@60Hz displays that you can use for working with various windows - while the laptop lid is closed - then you do not need such a setup. You'll just need to make sure that "Mirror Displays" is not checked in the Display settings in System Preferences.

If you want to be able to have 3 seperate displays, where one is the internal laptop display and then 2 external, 4k@60 Hz displays that you can use for working with various windows - then you do not need such a setup. You'll just need to make sure that "Mirror Displays" is not checked in the Display settings in System Preferences.

If what you really want is to have the laptops display and a secondary, separate workspace that is mirrored between the two external displays - that's not achievable with the standard macOS GUI. However, that would also be a quite uncommon setup.