How to blend two layers additive then override others

I have a AimLayer and a FireLayer needed to be blend additive and then override others. How to do this? I am using animator component.

Problem is when the player is aiming, the aiming animation overriding others is played with a mask of legs, so that one can aim while aiming. But if the player fires while aiming, I want an additive firing animation to the aiming, because I can aim up, but I don't have an animation clip of firing up.

The animator editor can't do this, they are either additive or override, and I need they are both override for others, but additive for themselves. Can I set in C# code?