Combining skinned meshes

What's the general workflow for combining skinned meshes?

I have an .fbx file with 2 characters. They have identical rigs, just their skin is different. One character is missing his arms and leg. The other character is just arms and legs.

In unity I'd like to combine these 2 skinned meshes. NOTE: I don't want to simply add the 2nd skinned mesh to the 1st skinned mesh's gameobject and have them share bones. I want to combine the actual geometry and then have that one piece of skinned geometry use the bones.

Here's what I've tried:

-Grab 2nd mesh

-Grab 2nd mesh's bones

-Grab 2nd mesh's bone weights

-Grab 2nd mesh's bind poses

-Add 2nd mesh to 1st mesh using CombineMeshes

-Append 2nd mesh's bones to 1st mesh's bone list

-Append 2nd mesh's bone weights to 1st mesh's bone weight list

-Append 2nd mesh's bind poses to 1st mesh's bind poses list

This, however, doesn't work. The geometry of the first mesh remains intact but the 2nd mesh's geometry goes crazy and basically all the verts explode.

What am I doing wrong?