I would defiantly try out a tandem with your wife before buying one. Thier are lots of places where you could rent one. My local city's rec department has a cruiser style one that my wife and I rented for an afternoon. We discovered that she liked it but not enough for us to get one for her and I.
I joined a tandem facebook group; let them know I was in the market for a used tandem with a budget of $1,000 and I soon had a couple of options.
I bought a very nice tandem for myself and my boys ages 6, 3, and 6months for $700. Found a friend who had a child stoker kit and now I have a bike that will last us for a very long time; but.... I may need a triple in the future.
Introductory tandem ride.