My suggestion would be to go and take a day-long intro to sea kayaking class before buying anything. You could do solo, or take your regular paddling partners. You will be learning on single sea kayaks, but the skills are all transferable to tandems. They should teach the basics of the kayaks and what the parts are used for, basic strokes, basic recoveries after a flip, etc.
After taking the class, then rent one of the kayaks you are interested in on a warm day in warm water and try the recoveries (you said the Old Town was in a livery’s fleet, so rent that, even though it is not something you are looking at now). See how they work. I suspect you will find what others are warning you of here - that self-recovery in one of the boats you mention here will be very challenging at best. If this is the case, you would really want to the use the boat only in places where you are close enough to shore to be able to swim to safety (when the water is cold, this is a much shorter distance than one would think).
For double kayaks that do have flotation, the Necky Looksha-T comes to mind. Also Necky Amaruck, but not sure that one is still made. These are regulars in tour company fleets, which I take as a sign of their value and durability.
You mention roof-topping - All of these boats we are talking about are total beasts off the water. Getting them on or off a roof will be very challenging. Only way to reduce this would be to look at a thermoformed kayak (such as the Delta 20T, which is some 16 pounds lighter than a Looksha-T) or composite (like Seward Passat). Thermoformed or composite tandems are likely special order items, unless you find used. Composite tandems actually are a bit too common used, as they are bought by couples who find they aren’t able to communicate well enough to make a tandem work, so end up selling them (why tandems are often called “divorce boats”).
A different possible direction - if staying in protected water close to shore is something you are willing to always do, you could consider one of the take apart kayaks by Point 65 (https://kayaks.point65.com/kayaks/modular). I haven’t paddled the sit inside versions, but they do appear a bit on the rec boat side, so not something I’d want to take into open water (with possible exception of the Mercury model, which looks like it has decent built in flotation). But you would get 2 large benefits - first that you could use these as single kayaks as easy as a double, and that you can take them apart for transport, so not have as much of a lugging needed to get a beastly double on to the roof of a car.
The brands outside of Necky I mentioned are somewhat niche brands, so may be hard to find. Delta and Point65 are both brands that REI carries on their web site, but not always all the models. You may have to contact the manufacturers to find out who the closest dealer is.