Blending 2 Caches - Issue

Hi all, hope you can help, I’m having a hard timing finding this answer on my own.
I’m trying to set up the left and right arms of my character in separate state machines so they can work independantly. My hands both have state machines like this


I also have a state machine for the spine which I added to my locomotion state machine as an additive. If I try a blend per bone with my SpineAnim Cache and Right Hand Cache, it works. The same if I replace it with the left hand. However, when I try to blend these two caches(arrows)

they don’t blend. The Layered Blend per bone only uses whichever one I plug in to the Blend Pose 0.
image

Any help you can offer would be really appreciated. Thanks so much.