If you choose the Modify option you can shorten your days, but you can't change to a split stay in one step. After you modify you can then book the other part of your stay as a new booking. Best to confirm your dates are available at the second resort before you modify your existing reservation.
Be aware you run the risk of your second resort booking up between the time you modify your existing reservation and then attempt to book the second. A workaround, if you have enough extra points, is to book the second part of your split stay, then modify your first reservation to shorten the days.