How well do you communicate with each other? Successful Tandem riding both facilitates conversation and developing an ability to anticipate/understand each other's actions/needs with only a slight hesitation in pedaling cadence. Or something like that. It has been said that, "Whichever direction your relationship is going, riding a tandem will get it there faster." My gf's first tandem ride was to learn how to use the drive train more efficiently, as her rides on her single bike exhausted her legs in a very small number of miles, while I was just getting warmed up. Since then, we ride the tandem exclusively, and the ride length is now up to 30 miles. The limiting factor is no longer her legs, but her discomfort on the seat. We're working on that with new seat, seat adjustments and coil spring suspension seat post.
I should add that the tandem is a mid/late 1980's Santana steel road frame, with a very small stoker cockpit (short top tube), 27 x 1+1/4" tires and bar end shifters. Today we test rode an mtb tandem, for the index shifters, wider gearing range, wider tires and the ability to ride gravel and mild singletrack. The wider tires are much more comfy, as is the better designed suspension seat post. She loved it and we placed our order. I have seen tandems on CL for <$1000 and for as little as $400, but be thoughtful before you buy used, and test ride.