You can't...well, unless you have a computer radio that can do flaperons. I have done this once with my futaba T-6XA. One servo is plugged into the normal aileron slot, and one is plugged into the flap slot. The radio automatically mixes the servos to give you normal aileron action, then then lowers both ailerons when you want. I am not sure about your hitech, so I can't say if it will do it or not. If it will allow you to, you abandon the y-harness altogether and set it up as I explained. Long story short, you can't have flaperons with a y-harness, just can't be done.