Difficulty blending separate arm and head anims together

I’ve got two anim montages that I’m trying to apply to the same skeletal mesh, one for the character’s head, one for it’s arm. Each montage is assigned to a separate slot. As you can see in the image I’m using a layered blend to merge the two for the final animation pose, but the problem is that the anim in the top slot is using the bone assigned to the second slot. So in this case, the Head anim is animating to the head bone as well as to the Right Arm bone. But this only happens to the pose on top, the Right Arm anim does not also animate along the Head bone.

So I guess the Layered Blend Per Bone node isn’t keeping the bones separate? I can’t figure out what’s going on and would really love some help if someone can tell me how to do this but not have the bones bleed into one another. Thanks.