I've never messed with the default UI since I think it's fine and it doesn't break when they patch something. However, having to click a hotkey every time you log in to get your target-of-target window back and then having to reposition it and set it's alpha settings is just too annoying, so I added it to my regular target window.

This is a replacement target window in the same style as the default UI that shows both your current target and your target-of-target window at once. This will work automatically whenever you have access to this ability due to your group or raid leader having it. If you don't have access to the ability, it shows only your regular target.