Why does my multi parent constraint not work with my animation?

So, I've got an animator controller with two layers. I've set one layer to hold an object an animate the hands holding it, and the other contains the animation for running.

I've set the appropiate masks like this:

The Upper body mask is the opposite.

My base layer where my running animation is located contains of course the lower body mask and the second layer contains the upper body mask.

I also have an multi-parent constraint that references the left and the right hand.

On the left you can see the upper animation and on the left you can see the result of the animated layers. What am I doing wrong? I am completely and utterly lost. Thank you.