Interference between Render Layers?

I’m setting up a motion-matched composite using Render Layers to separate the meshes that are used for the “beauty” pass and those used for shadow-casting.

Below are some stills to illustrate a prob I’ve run into:

http://img146.imageshack.us/img146/8525/rlglitchzz8.jpg

Top L: This shows the meshes used to generate the shadow elements. These occupy one Render Layer, the “Shadow RL”.

Bottom L: These are the “beauty” pass meshes, fully-textured and lit, on the “Beauty RL”.

Top Ctr: This is the “shadow” pass I want, to be combined with the Beauty RL and live-action BG using a Color/Multiply node. I can only get this if I render the shadows without rendering the Beauty RL at the same time.

Bottom Ctr: This is the shadow pass I get if I render both the Beauty RL and Shadow RL at the same time, which is the only way to composite the shadows in one render pass. Note the glitchy shadow pattern that appears on the pillow in the FG. This only shows up when both the shadow-generating mesh & beauty mesh are rendered together, even though they’re on separate Render Layers.

Top R: Live action & models composite without shadows.

Bottom R: Composite with shadows added in single pass – note how the gltichy pattern messes up the pillow.

I could render the shadow pass as a separate sequence and then add that back in as another composite node setup, but I’m wondering why the glitch appears in the first place. I was under the impression that meshes on separate Render layers can’t interact with one another, but they do seem to be doing so in some odd fashion.

Any ideas?