You could try a geometry shell. I’d even set the offset to 0. You’d use an opacity map for both the ‘robotic’ shell and the ‘human’ shell to mask out the other side. Then you could use separate shader definitions for each.
That idea was so intriguing, I had to try it out, so here’s a quick test render. The skin part uses the Skin lighting model (duh) while the metal parts use the Glossy Metallic lighting model. Lights are one of Lantios’ sets.