+1 on checking the headset. Wear or incorrect adjustment there can really goof up steering.
...owner was a bike shop owner...
Maybe contact the previous owner, not for a refund, but for advice.
Hopefully you'll get this straightened out, and you'll be able to get started having a great time riding with your girlfriend! Here's some reading material that may help you get started:
http://sheldonbrown.com/tandem.html
http://www.tandemcycleworks.com/firs...irst_ride.html
http://www.gtgtandems.com/tech/propmethod.html
http://www.rodbikes.com/articles/web...ngstarted.html