As mentioned, tandem kayaks are heavy. Most ppl prefer the independence of their own kayak. Locally, though there are couples who enjoy the Hobe Mirage tandems in our local outings. They are also good if you want to take along some grandkids.
Hobe also makes a small kayak that a kindergarten kid can launch and pedal/paddle. There is a clip on their DvD, maybe site.
You can check out U Tube search for "Adventure Island Kayak" , the tri-hull, or just Hobe Kayak to see them in action.
I think you ought to check out the Hobie Kayaks with the Mirage pedal drive. It has rubber fins underneath that work like a Penguin's wings. It is much easier than paddling and they have a rudder and you can easily get a sail. They come in short, medium and sea kayak configurations.
I have the three-hulled, Adventure Island model which has a bigger sail setup than the single hulls. As mentioned, the Yakima "Hully Rollers" will help you line up the kayak for roof tie down. You only have to lift half of the kayak's weight at at time to get it on the roof and your boat's bow pulpit or aft railing makes a good half way pivot point.
I use my kayak for hours at a time with little tiring since it is easy to switch from the traditional paddle, to the Mirage pedal drive, to the sail. They have a wheel set which stows inside the kayak and can easily be plugged into the hull. Lots of storage and fish rod holders are built in.
John