It was commonly done in the mid 60's when a lot of the proportional radios only had three channels. Back then it was refered to as "CAR" which stood for Coupled Aileron and Rudder. But if your radio can do it, it is better to mix them together so that the rudder is independent and when you give aileron, it mixes with the rudder for better turns. Depending on your airplane, if you want to tie the two together with a "Y" harness, I would keep full aileron travel with about 1/2 rudder. In the air, this will have the effect of speeding up the rolling rate.