Did the two of you work well on the tandem? Some couples don't or can't handle riding a tandem well. One wants to ride faster, the other wants to ride slower. One wants to spin, one likes to mash. etc.
If your wife finds that she likes riding it may be for reason that differ from your own. You may love the speed, she may just want to smell the flowers.
Even if you get a tandem, you can still ride the Bianchi. My SO (soon to be wife) loves the road tandem (recumbent) but shes not so hot about the off road tandem (upright) for the same reasons, seat pain. But with a little give and take it all works out. Myself, I enjoy all forms of cycling so I'm happy riding whatever.
If she liked the bent, get her the bent. you can still ride together, her on her bent and you on your Bianchi. Just try to keep up with her
If you get her a bike she dose not enjoy, she will not enjoy riding.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." - Arthur Conan Doyle