Layering morph/blendshape animation

We are trying to layer on facial animation on our character over an idle anination. The idle has facial animation and we can use Layer Blend Per Bone to mask over the facial joints, but the blendshapes seem to be fighting each other. Seems to try to play both blendshape at the same time. How can we layer of each blendshape animation over another like we can for joints?