iOS 12 gives you the ability to save two faces with Face ID.
Update comparable to Touch ID
Touch ID allows you to store up to 10 fingers. However, this was never intended by Apple to allow 10 different people to access the device.
The same is true for the upcoming update for Face ID. You can then add an “alternate appearance” and also do facial authentication. This is actually intended for people who have two very different “manifestations”. Imagine, for example, actors who look quite different with glasses, hat and mustache on the set than they normally do.
Of course, you can still use the feature to give your spouse or child access to the device allow to enter without a pin code. How this works is shown in the following video.