If you and a partner plan to paddle together on a regular basis, and you enjoy the proximity, then there are several advantages to getting a double kayak rather than two singles. For one, a double kayak is usually cheaper than the equivalent two single kayaks. Loading one double kayak onto your car and finding storage in your garage etc. are also advantages. Other people may decide to get a double rather than two singles because perhaps one of you is a stronger paddler than the other, for taking kids or dogs on the water etc.