Keith, I am not familiar with that TX so cannot tell you the particular steps but most TX will have either a separate mix just for this or you will have to go into your P mixes and assign the two channels you want mixed, the amount of mix and the directions.
That being said, I am curious as to why you desire such a mix as it is useful/required on only a very few models such as full house sailplanes or a Cub type airplane.