I have been printing duplex organizers for several years now. You will need to add in (force) a second page to print on the slip sheet.  As suggested by George4Tacks, add enough carriage returns to bring you to a second page and then add a simple period "." to that page so the editor recognizes there is a second page to print.  The dot is small, negligible toner is used and it should accomplish keeping the slip sheet separate from the letter and tax organizer pages.  Hope it works for you. 🙂