I would recommend riding that bike for a year or two with only a few changes. Learn what you want in a tandem before making a big investment. We started back in 1973 with a Gitane we purchase from a teenager who had ridden it across the country. He was well supported by rich parents, which meant the bike came with lots of spare parts. It was a lousy frame that was so flexible it was difficult to stand while climbing. Still, spending two years on that bike taught us what we wanted in a tandem.
Solid axles were standard issue, even on good tandems, in the old days. Partly this because quick release skewers long enough to fit 135mm axles were hard to come by. As someone already noted, tandems brake axles on hubs designed for use with threaded freewheels; however, long replacement axles are still available on the Internet. We always carry a spare when we tour.