https://www.cnet.com/news/ios-12-adds-face-id-support-for-a-second-appearance-says-report/

Add the ability to recognize a second person with Face ID authentication to the list of new features in iOS 12 that weren't announced in the WWDC 2018 keynote on Monday.

That's according to a report by 9to5mac, which found the option to "set up alternate appearance" in a beta of the new operating system software for iPhones and iPads.