The duet display supports a Touch Bar, and when mirroring the display, due to the different screen aspect ratio between iPad and MacBook, there will be a black empty area on the iPad. The duet display takes a use of the empty black area to place the Touch Bar, I hope luna display catch up at this point. Thank you!
Luna will not have any black bars, simply because it will have its’ own apect ratio since it is (or at least behaves like) a separate screen.
Luna does not have any TouchBar support to my knowledge.