When iPhone X first came out with iOS 11, users were able to register only one face to use with Face ID but with iOS 12, Apple adds support for two Face ID where you can now register two different faces or register one face two times, in case Face unlock fails.
Apple mentions that, this option will come to handy to users when Face ID fails to recognize you when you put on sunglasses or a beard. Or you can just assign two different faces that will work perfectly to unlock iPhone X.
This is definitely a small change but this option will now make Face unlock more accurate and reduce Face ID fails.
Many new features were introduced in iOS 12. If you haven’t checked iOS 12 hidden features then go through this posts.