As Scale only noted the simplest way to set up a second rudder servo (makes no differance whether or not the second one is for air or water) is to simply use an ordinary Y chord on your channel #4 One leg going to the normal air rudder and second going on down to the water rudder.

You could on your radio slave another channel to your rudder #4 and use another separate extension from that channel on down to the WR. Regardless there is no advantage either way and you will still need a form of mechanical adjustment on the Wr. Either way you will still have to have an extension down to the WR regardless.

My preferance is to forget the separate servo and just use a long flex line from the opposite side of the rudder forward and bending around to near the trailing edge of the wing and back to the WR to hook up mechanically. Far easier and most important very easy to adjust out on a beach somewhere since water rudders are always getting banged around during operations. This is the setup I use on virtually all my float planes because of its simplicity and its functionality in use.

Note the simple cable in the pic from the air rudder to the water rudder. For the air rudder a second control horn is just installed opposite of the other side:

John