[Joints] How to set connectedbody using raycast ?

Hi, I'd like to connect two rigidbodies together but I'm facing a problem :

Raycast ---------------------> ◘ ◘²

What I want to do is set the second cube(²) as connected body to the first cube.

Example : I want to nail a wooden plank to another one located underneath the first one.

1) I aim at the point on the first wooden plank where I want the nail(joint) to be.

2) When I click, the joint is created and the second plank which is underneath becomes the connectedbody to the first one.

I've done the first step above but I don't know how I can implement the second one. Could you help me please ? Thanks a lot.