Be very patient! Please take short rides and ask her what she likes. Communicate! Communicate! Did I say communicate. Let her know when you are shifting, let her know when to coast, determine which leg to start coasting on, give her plenty of warning when a bump is coming. Develop a starting and stopping technique that works for you!
It can take time but for us it has been well worth it. Make sure that she is comfortable, the best money that I ever spent on the tandem was getting a professional fit.
It might be good for you to both take a spinning class at the local gym.
By all means make it fun. My wife loves to ride the tandem but we do not "train", we ride, sometimes very hard and sometimes very easy, but for her it must be fun.
Our combined age is 139 and we try to ride every day that it is above 35 degrees outside and when it is colder we ride indoors on the Computrainer.