https://www.theverge.com/2018/6/5/17428636/ios-12-face-id-two-faces-support-alternative-appearance

Apple appears to be readying multi-user Face ID support in iOS 12. 9to5Mac reports that the recent iOS 12 beta lets iPhone X users setup an “Alternative Appearance” in Face ID settings. “In addition to continuously learning how you look, Face ID can recognize an alternative appearance,” reads Apple’s own note on the feature. Touch ID, used on older iPhones, supports fingerprints from different people, but Face ID has been restricted to just a single user so far. While the new iOS 12 feature is clearly designed to recognize a different appearance (possibly for hats etc), testers have found it works on an entirely different face.

9to5Mac speculates that this feature could be related to an iPad with Face ID that’s also referenced in the iOS 12 beta. Several reports have suggested Apple is readying a new iPad with Face ID and no home button for later this year. An additional face for Face ID might be useful for a new iPad, but it’s also a feature that many iPhone X users would likely want to see too. Multiple faces support for Face ID is particularly useful if you share a device with a family member, or for potential new iPads in an education environment.