I want to use liquify tool functions to edit a face to have features of another face that is on another layer.
An example may be that I want to drop the opacity so that I can magnify or move the nose around to be like the nose of the other face that is on the other layer.
Let's assume you have two layers, Layer 1 (the image you want to match), and Layer 2 (the image you want to liquify). Obviously before you start make sure both faces are the same size and in the same general position.
Select Layer 2 in the layers panel, and click Filter > Liquify
When the Liquify window opens, scroll down the options on the right, and choose Show Background. Set the Use option to Layer1. You can change the opacity by moving the slider, or you can toggle backwards and forwards by selecting/deselecting the Show Background option.
This example is for Photoshop CC 2017
If reducing the opacity in the layers panel do not work because you want varying opacities in different places, add a layer mask. The darker is a place in the mask, the more transparent is that place in the image. You can spray white or black or erase something in the mask on the fly or temporarily disable the mask.
It also can be useful to try different layer blending modes and to adjust temporarily (=with adjustment layer or making a copy) the color and contrast of the model layer.
If you want to make an exact copy of some detail, you can also copy it and match the color.