I think some of these replies reflect a widespread and somewhat thougthless prejudice towards tandems. If it turns out you and your partner have widely different paddling abilities or just styles then paddling two singles together will be misery. I speak from experience, myself and my wife having started out with Guillemot and Guillemot S singles. I would end up too far ahead then doubling back, nagging, and spoiling what should have been a happy day on the water. What social nature there was between us was not good but mostly our boats were not close enough for any socializing anyhow.
So I built a Cape Ann Double. Everything got better. Sure, as Scott says, the feel is not like the Guillemot, I liken it to an oil tanker, but again, as Scott says, it is more efficient. Even though my wife is not a strong paddler, we can go as fast or faster in the Cape Ann than when I paddle the Guillemot single and my wife stays home. Of course tandems are heavier but two people your age can easily manage a well made one. Of course we think twice about paddling up a river that might turn out to be too narrow to turn around in but you are talking about ocean paddling. We paddle the Cape Ann on a huge lake (Google Lake Winnipeg) and find it very stable in rough water. The placement of the rear cockpit means that if you and your partner have different paddling patterns, you will not clack your paddles. The Cape Ann has enormous capacity if it turns out they you do multi day trips.
My only negatives are storage, car-topping and turning. Storing a 21’ boat can be an isssue. Car topping is a bit of a grunt if you have to lift it onto a high roof and I would prefer a roof that is flat and long. As it is we have a Prius. The roof is low but short. That being said, we use bow and stern ropes to the four corners of the car and have car topped it from central Canada to the west coast with no problems other than looking a bit ridiculous.
As for turning and manouvering, a 21’ kayak turns a bit like a Greyhound bus so a bit more planing is necessary at times.
Summing up, you said you were going to build a single as well as a tandem so that pretty well takes care of some of the issues mentioned by others. I think a lot of the negativity around tandems comes from people’s experience with short, cheap plastic ones used by rental outfits. There is a world of difference between paddling those and paddling a longer well-designed double.