I fly bipes all the time and when using 4 aileron servos I use two matchboxes to ease the set up and save programming functions at the transmitter for other uses. The right wing will plug the matchbox into the aileron channel and have two servos plugged into the matchbox.

The left wing will have a matchbox plugged into aux 1 or channel 5, and have two servos plugged into the matchbox. If you are really into some very exotic mixes, then you may want to use multiple mixes and separate channels for each servo. Using "Y" harnesses limits your ability to match up the upper and lower aileron for each wing.