Generally you go to channel 6 or Aux1 for the second aileron channel.
If you also have separate flap surfaces. desiring the multipoint or slider settings for flaps and don't want flap mixed to the ailerons than you can use the "gear" channel or channel 5 This would require a programmable mix since its not the way any pre-programmed "flaperon" mix works.
You could actually use any AUX (ch 5 or higher) port for the second ailerron. There's nothing really wrong with this.
In some advanced models when using a 9+ channel computer radio there will be 4 or more aileron servos each on its own channel.