What's the story behind the highly detailed Spy face unwrap?
The unwrapped Spy faces are from his 3d model. The reason why there are two of them is because this allows more versatility in showing emotions than if there were only one single "skin".
in order to show skin folding when the models are posed to show different emotions, without painstakingly 3d modelling every individual wrinkle in Spy's face, the use of a two dimensional skin with heavier shading can be faded in and out as needed when a certain slider is adjusted.
Think of it like how makeup artists heavily contour the cheeks of their faces. With the simple application of a pigment emphasizes structure, shape, etc. And in the case of Spy's model, although his 3d model does wrinkle a bit, this effect bears the brunt of conveying a new shape to the viewer.
You can clearly make out how the second "skin" fades in and out alongside the deformation of the face as I slide the slider back and forth
