How to combine upper and lower body animations into one?

I'm having trouble "splitting" character into two parts - lower upper body.
For example I have "idle" animation affecting both lower and upper body but say character aims and so I want to change upper body animation to shoot while keeping legs at idle.

I thought of using avatar masks for that but I ran into a problem.


Left: Animation controller only has default layer with Idle animation in it.
Right: Two additional layers (upper lower) each controlling own parts of the body. Character appears angled for some reason. This happens on all models I tried, not only the robot one. (default layer is empty in this case)

The lower body mask has legs, leg IK and root shadow in green
The upper body mask is opposite of lower body mask

If I change leg animation to crouch it will also appear angled and totally not what I want.