Question How to apply same animation in two different but same father object and one object has unique layer?

like this video:

There is two objectne is the gun(with bone) and one is the hand(with bone too).Two object are in the same fbx file
First after i finished animating,they exported in the same fbx file sharing the same animation from 3dsmax or blender or something.It works fine as excepted.Hand bones moving as except,gun bones same.
After that i deactived the hand skinned mesh,and export skinned hand(and the bone attached on it)as individual to unity.
I watched this post:
https://forum.unity.com/threads/animate-object-when-its-animator-is-in-different-gameobject.462995/
and i found i'm encountering pretty same situation with him.But the problem is:I already animated 5 weapons and it is impossible to do it from start.Six years later maybe there is a solution i guessed?

Also,even if i restart to animating it.I still don't know how to give different controller a same layer in animator during the runtime.For example:lots of weapon have it's own unique animation and it's own Animator Controller,But they all have the"left hand throwing grenade"animation(just like the video shows).How do i do this?Animating them each one by one,or there is a way to add layer to animator controller during the runtime?

Thank you

 

Guys,I figured it by myself.
What i need to do is to separate two arms,one is called"left arm"the other is called"right arm".
When I throwing grenade,I'll just deactive the left arm,and spawn a new left arm to throw.After the animation is complete,I'll re-active the left arm.And the problem is solved!How good is that.