Although dual-display handsets never managed to take off properly, we keep seeing such designs showing up from time to time. The upcoming 6.26-inch ZTE Nubia Z18S is expected to feature a 5.1-inch secondary color display on the back and premium mid-range specs. Its recent stop at TENAA suggests that the official announcement is near.
https://www.notebookcheck.net/Dual-display-ZTE-Nubia-phablet-hits-TENAA-short-hands-of-video-also-available.334280.0.html