When you are rendering the human figures separately I freeze all other layers and then I render the scene. But I wonder what happens with the lights. Do I have to unfreeze its layer or it doesn't matter?
If you would render the silhouet people seperately from the whole model, you'll end up seeing parts of them that should be hidden by other elements, I would say.
I think there are some shader options for 'cel shading' style surface settings for cinerender, but I recall a topic about this a while back where in the end it doesn't seem to be working in cinerender like it does in C4D.
Maybe a quick effect trick in photoshop will be the fastest fix for you.
Sometimes getting everything correct in the render (no post production) leads to renders that take forever, whereas the post work would take 2-3 mins.