https://www.cnet.com/news/forget-about-dual-cameras-this-phone-has-dual-screens/

This kind of thing has been done before, kind of. In 2014, the YotaPhone 2 came out of Russia equipped with a standard front LCD and a secondary E Ink display on the back. It was designed to let you read e-books for long periods without draining the battery. But the E Ink technology wan't great, and the phone was too expensive to become anything more than a niche device. On paper, at least, the Nubia X appears to have more potential to help dual-screen devices hit the mainstream. 

The Nubia X has a 5.1-inch rear OLED.