Just call them and request it, they'll make a note in your dining reservation and try to accommodate you, which they usually do. Also if the first night you get to dinner and you are seated with anyone, you can usually check with the Head Server and they will try to reseat you for future nights since there are so many changes the first night with people switching seating times and restaurants. Also it's nice to note that Disney tries to match the age profiles of people they sit together, so you're not going to be placed at a table of a family with kids, it'll most likely be another couple traveling by themselves.