Unless you have an annual pass or one of your stays is at DVC on points, yes, you would have to purchase tickets for both stays in order to add the dining plan. As for whether you'd be better off, try doing the math for what it would cost to put tickets on both reservations. Remember that you can buy length of stay tickets with your first stay and buy a one-day base ticket with your second stay. One-day tickets never expire and they can also be upgraded or used toward future ticket purchases.