I also agree with YNOT,

Doing this with a Y cable would be bad. If you are looking at having the rudder move with the aileron you can use a computer radio to mix a small percentage of rudder to the ailerons. Cubs need this to fly the best and it is called coordinating your turn

To be really honest here. You should learn to coordinate your turns before mixing it in with the radio. Learning when and how to use the rudder is a good thing and will help to make you a better pilot and you will soon see that mixing for coordinated turns is a moot point

Now for Knife Edge flight, mixing rudder to elevator to mix out a pitch change is good because it can help take some of the load off the pilot, especially on a biplane where you almost always need to mix rudder to elevator and rudder to ailerons.

I am not putting down mixing, I am justr recommending that before you setup the mixes you learn a little more about ALL of the planes control surfaces.