Multiple Fadings atop one another

The title is probably confusing but I will simplify it. Basically, I want stuff to fade in/out on top of each other.

E.g. You know how when you animate a movement, you ‘actually’ animate it? Well there is this other technique in which instead of ‘moving’ the object, you create the different movements and fade them into the other.

So let’s say there is a person staring in front of you. You want him to turn his head to the right. Instead of ‘moving’ the head, you create both images (one facing towards you and one facing to the right) and make the Front face fade out while the Right face fades in.

This is what I want to be done but I can’t seem to do this because one or the another, one of the image will cause the other to disappear and as you know, you can’t fade in a mask.

Does anyone have a solution to this?