The overall body armature is top of the list. It has a neck bone that deforms the entire head as a unit at about weight 0.50. The Face Armature is a child of this bone as well, so when the head turns the Face Armature follows. The Face Armature root bone has the Eye Armature as a child, and this controls eye rotations. The reason for this Russian dolls approach is to keep each armature from becoming too complex and to divide the influences into “specialties.” It also makes adjusting the rigs to account for character difference easier, important for my Universal Figures concept.
In general the bones controlled by one armature don’t share any influence with those of one of the others, making weight painting much easier, but it is possible. I do not use Auto-Normalize, since that would mess up such shared influences.
For example, in my female rig I had to develop another specialty Armature, only three bones, a Root & 2 deformers. Because I use Shrinkwrap to conform the breasts to Soft Body objects in the muscle system, it tends to flatten the nipples completely, looking a bit odd, so I reduced the Shrinkwrap weight in those two small areas, but then the nipples don’t follow the Soft Body motion well, looking even more odd. The specialty armature uses Envelopes rather than Vertex Weights, and its 2 bones are constrained to follow two empties vertex parented to the SB breast objects at the nipple region. Thus I get the Soft Body motion without needing the same Shrinkwrap weight for that small number of vertices. A little extra work, yes, but it solved the problem. BTW, using bones in the main body Armature to do the same thing led to cyclic dependencies that messed up everything, so a separate Armature was a necessity.
Having two armatures in the stack means you have to establish the proper hierarchy, with one being the “master” and any others subservient to it somehow, through parenting in most cases (the nipples armature is parented to a spine bone in the main body armature). Can’t parent the same mesh to two armatures, so instead you pass the parenting “down the chain” so to speak, and keep vertex weight influences separate whenever possible.
PS. Just read DanPro’s comment & I agree 110%, maybe even 115%. Multiple armatures are possible but not always a good idea. For me it provides adaptable solutions but that doesn’t mean it is optimal for every rigging problem.