First: test ride a tandem.
Prices on tandems vary from Wallmart (not recommended @ $175) to $13,000+ (yes, that's 3 zeroes after that number 13.
As with singles there are choices between road/off-road/hybrid/recumbent . . . and 2 and 3-wheeled tandems. There is steel, aluminum, titanium, bamboo (!) and carbon fiber (or combinations there off).
Is your wife even interested in going on a tandem . . .
There is a learning curve to riding/handling a tandem.
A sprung Brooks saddle works for you on a single. Does she ride a sprung saddle also?
Everbody's butt/sitbones cannot handle riding for hours on end.
As the old IBM slogan said: THINK!