I once answered it but then deleted to correct my answer. after all I just got how to blend aim offset with just one montage not more.
so I blended aim offset with a shotgun shooting montage and my character is able to walk, aim and fire shotgun at the same time all correctly. I did the following steps.
1- cached the state machine pose (the main pose, probably including your “idle/Walk/run” states).
2-then I used "use cached pose (the main pose) and dragged a wire from it and typed slot and set it to the montage(shotgun_shooting) and then cached its pose.
3- I blended the second cached pose(step 2) with aim offset.
4- I blended the first cached pose(step 1) with aim offset’s right side pin by using “layered blend per bone”
5- then I set bone name. and set blend depth to 1 and checked “mesh space rotation blend”
