If you purchased the VISTA as a single, you'll need to make a couple of purchases to convert the stroller to a double. The single VISTA comes with a Toddler Seat that fits only on the upper position of the stroller. You'll need to purchase the RumbleSeat, the smaller seat that fits only on the lower position. The two seats are not interchangeable. You'll also need adapters to enable most double configurations. The upper adapters raise the top seat higher, creating the needed space for two seats. You'll also need the lower adapters if you want to attach a car seat or bassinet to the lower position. See How to Convert the UPPAbaby VISTA to a Double Stroller for more information.