You have 2 servos and 2 articulations. Attach the second U-bracket to whatever you want to . Notice that it is also drilled for attachment to another servo. How many degrees of freedom/movement do you want? It's your design.
That looks like there should be a bearing, and so that should be a shoulder screw, or possibly a whole bearing assembly, since that is aligned with the rotation axis of that servo module. It needs to allow the two brackets to move relative to each other. So at this point you really need some application information about that particulare servo module setup. Anything else is a guess.