Trying to "stack" layered blend per bone

I’m trying to use layered blend per node to control the right arm with two different animation montages. One montage moves the arm (aiming a gun) while the other closes the fingers (holding the gun). The montages have “Right Arm” and “Right Fingers” slots respectively. I’ve tried about everything I can think of but I can never get them to work simultaneously. I can activate one or the other at runtime, they each look as the animation montage appears but they do not stack. Can anyone tell me what I’m doing wrong?

Fingers are not closed, its only layering the arm animation.

I don’t think you can stack montages. I’m pretty sure you need additive animations on the same bone.
Although I don’t know very well why you use montages for that, for looped animations, isn’t normal animations better?