Test ride/rent/borrow a tandem first. Have someone who is a tandemist show you how to mount, ride a tandem. First ride it solo, so you learn how to handle a l-o-n-g bike.
If possible, ride with an experienced tandem rider, both as stoker and then as pilot. Only when you gain some confidence should you put your wife on the bike.
Key to riding as a tandem team: Communicate!
Pilot/captain (the person up front) must tell stoker/rear admiral (the person in the back) what they are about to do. Voice the command: shifting, braking, coasting, pedal, coast, stopping, slowing, etc.
Stoker does all the signaling (left, right, slowing) upon captain's request, that way you keep your hands on the bars.
Stoker feels totally helpless as she can't really see what's in front of her and has no control whatsoever.
However it relieves her of the responsibilites of steering/braking/shifting.
If you take it easy and ride predictably, she'll readily take to riding 'in tandem.'
We've been doing it for 32+ years and many thousands of miles, and are still happily married!

Pedal on TWOgether!
Rudy and Kay/zonatandem