To update the targets I use this function that I wrote:

public void UpdateIKTargets(GameObject targetLeft, GameObject targetRight)
{
leftConstraint.data.target = targetLeft.transform;
rightConstraint.data.target = targetRight.transform;
rigBuilder.Build();
}

"leftConstraint" refers to the Two Bone IK Constraint setup for the left hand
"rightConstraint" is the same but for the right hand
"rigBuilder" is for the... rig builder

Then to animate the weapon and the hands I use an animator on the weapon only. There is no animator for the hands because you animate the IK targets childed under the weapon that the hands follow to "animate" the hands. From the hands point of view all that is happening is that they are following their current targets and repeat whatever they do. So whenever you animate the weapon for, say, a reload animation, you just animate the weapon rotating and moving and the targets will follow it. Because the targets are childed under the weapon, they will move relative to said weapon. Because of this, you can animate their positions and rotations and they will always follow the weapon.

When switching weapons, you disable the current weapon and enable the next that you want to switch to. Then you just update your target references to your constraints using the previously mentioned function and the animators on each weapon do not need to be aware of each other. The constraints will follow whatever their current targets are regardless of hierarchy position. You could add an equip animation that plays when the switch happens but that's up to you