When you edit your 'Sitter wanted' listing (available from your Dashboard), under the 'Dates' section, just click the 'Add a new date range' button to add a second or third set of dates. This will allow you to select a second (or third) house sitter for those dates.
Important note: To view who has applied for which set of assignment dates, visit the 'Manage my sitters' page (from your Dashboard). You can manage everything for each set of dates from this screen, and is seperate to your 'listing' page.