My fiancee/stoker hadn't ridden in 15 YEARS before we got on a tandem... She had a crash in high school where she was hit by a car and needed facial reconstruction, the whole 9 yards. I was a pretty hard-core cyclist, and we decided to give tandeming a shot as a way for her to get into riding where I could take care of all the traffic, braking/shifting/cornering, etc.
Our first test ride was at a shop that had 1 tandem in stock. It was the wrong size and we got NO instruction on how to ride it. After a brief stint we decided to go to a good tandem shop.
The next test ride was almost disastrous. We were making a left turn and our communication wasn't so hot yet, so I thought she had told me we were clear in back. I started to turn and found the car that was coming up on us--fast. I swerved and missed the car, but she was FREAKED OUT given her history...
After deciding to go eat some lunch, we came back to the shop and rode again. This time it was fun, and she found herself really getting into it. We rode 5-10 miles until we came to the realization that they didn't have the bike we were looking for in stock. We went and did some online searching and found Mel at Tandems East, who had the tandem we were looking for in stock.
The next weekend we went to Mel's and test rode the Burley Tosa. We put about 20 miles on it and took it home. The next day Mollie and I went for a 25-miler. After that ride, she was clammoring to do the half-century that was scheduled for the following weekend. Within 3 weeks of owning the Tosa, we had ridden a 67-mile tour.
So, the moral of the story is, BE PATIENT, and remember that your single bike ways are no good on a tandem. You have a passenger now, and she will most likely be a little freaked out at first. You just can't ride as aggressively on a tandem. Period.
The other key is to go to a shop that will take the time to teach you and let you try out a couple different brands/models. Without that I'd bet your tandem experience will be a rougher one.
Good luck to both of you! It CAN be done!
DrPete