For the extremely upright riding experience, the only configuration that will really cut it is the "beach cruiser" style of tandem that are exemplified by the Wal-Mart Dual Drive, among others. Electra also offers an upright tandem that will likely be better made than the Wal-Mart offering. We examined this bike but I couldn't honestly see any real difference between it and the several cheaper models that cost 1/4 as much. Of the el cheapo pack of cruiser tandems looking for impulse buyers, we can vouch for the Wal-Mart version. We are on our second. We gave the first one away when we moved cross country, and promptly bought another in our new city. That was five years ago and it is still going strong. It is extensively upgraded. Only the frame, handlebars and the drivetrain is original, and even that has had the bottom brackets upgraded to cartridge bearings. It is a 60lb beast even with our upgrades. We have a Bob Yak for shopping (we are car free also) and the tandem is equipped with special axle nuts that receive the trailer. We routinely pull loads in the 120's of weight but cannot in good conscience advise towing large dogs around... unless you are towing them to the beach where they will then get a run in. But that's us. Two hours in a kid trailer is not at all like being in the SUV. The dog will hate it. In any case you would definitely feel the frame flex when a captain and stoker of moderate weight get a 105lb load attached.
We are in our mid-fifties as well. Pretty fit. If the Wal-Mart were our only tandem we would feel pretty limited by it. You may find you want to join a tandem club and ride with others. Don't even think about bringing a beach cruiser type rig to a group ride. Our Raleigh Coupe flat-bar road tandem was only made for two years. Raleigh still offers the Companion which is not a road bike but most of them probably are ridden entirely on road. We see many, many conventional tandems adapted to older captains with bad backs. For an entry level you are going to see makes like Cannondale a lot. The Burley's and Treks that were mentioned don't seem to turn over that often and when they do, its on consignment from someone knowledgeable and they will be motivated to secure the highest price possible for their clients. On Craigslist we see mainly the usual Cannondale, Co-Motion and Santana and of course the Cannondales will be the most affordable.
Our Wal-Mart tandem is five years old now and our Raleigh Coupe is eight years old. However, at the time we bought them they were brand new i.e. state of the art. We could have taken the advice we received at the time to buy seriously older but high quality tandems from knowledgeable sales representatives, but this would have meant going backwards decades in performance features. We did not consider it advantageous to go back to friction shifting and 5 speed freewheels when for a similar amount of money we could have the latest in index shifting, 8 speeds (9 speeds!) and disc brakes even, in the case of the Raleigh.
Bottom line: for a couple such as yourselves looking for a nice rig to ride for a couple to three hours and not to be a car replacement, we would skip the beach cruiser utility bike stage and go directly to a used Cannondale off Craigslist. Be careful buying used specialist machines from members of tandem clubs. They paid a lot for those bikes, as in most cases they were custom purchases. You may find your budget going out the window. Speaking of budget. Have you decided on one? That would be helpful in steering the rest of this discussion, I think.
H