I'll pass on the advice/lecture the tandem shop owner gave me and my wife when we got our first tandem.
"If you want a riding partner for a long time, do the following. Tell her when you are shifting. Tell her when you are braking. Ask her if you are going too fast, following someone too close, etc, and respond accordingly. Give her a crotch break all the time. Have a destination in mind when you start out (coffee shop, etc). Gradually extend the distances."
It worked for us. My wife is very comfortable, even at 60+mph (if the road is clear and smooth)
Cheers