One of the limitations with attraction pre-selection, initially at least, is being able to split your party when selecting your pre-arrival attractions.
You must initially select everyone in your group to ride the same attraction simultaneously. Once the reservation is confirmed you can go into your plans in the My Disney Experience app and amend either the return window for the attraction or select a completely different attraction within the same park. This can be done on a person-by-person basis if you are managing a group of people.