I used Et Futurum Requiem, which allowed me to render a double skinned body in 1.7.10, like this:

But when I add MPM or MPM+, it won't allow me to display double skin：

In Et-Futurum config you will need to disable 1.8 skins! Go to MPM+ and change your Model Type to Steve64 or Alex then go into BODY/ARMS/LEGS/HEAD configs and you can enable your 3D Layers!