So this is a little trick I use to make things a lot simpler in my renders. Start by applying a Geo shell to your character (create -> New Geometry Shell), switching it over to iRay (first image), turning everything to Cutout Opacity zero, then turning the Torso on to 100% and applying the torso opacity map (separate file uploaded to Scraps). You can now see the effect in image 2. I can then turn on the legs and toenails below this and change the Surface settings for a nice effect (image 3)!

If you want to combine elements from 2 different models, you can try applying one model to the geometry shell first, but this doesn't always work (some REALLY want to only apply themselves to the base model underneath). If it doesn't work, you can just load the 2nd model into the scene as a separate character and then apply the same maps and settings across to the Geoshell manually (time consuming, so save the resulting file for future use!) to get exactly the same look, then delete the additional character. You can then apply the opacity maps and get a combined/mixed render character.

Have fun!

Oh, and I like to change my Geo shell "Offset Distance (cm)" from 0.1 to 0.02 as it makes the transition look smoother. Of course, this will depend on how much bump mapping, normal and displacement settings I'm using on the shell as too much difference in the shape of the shell could see the original model poke through.