Here's my .02 worth. Tandeming (is that a word) can either be the best thing for a marriage, or the absolute worste. I can honestly say some of the best times I've had with my wife have been on our tandem. A few years ago we rented a tandem when we were on vacation on Mackinaw Island. We really had a lot of fun. So when we got home, I bought a cheap one from a guy who was having a garage sale. I stopped and asked the guy how much he wanted for it, and he said it wasn't in the garage sale. I told him my wife wanted it so it now was in the garage sale. Needless to say, he sold it to me for a couple hundred dollars. We rode it a few times and then upgraded to a decent Cannondale.
So where am I going with this story? I learned a lot from riding a tandem with my wife. Now let me say that my wife really wasn't into the training part, so your wife may be different. We just wanted to get some exercise and spend some time together. I thought I would be getting some good workouts, but it turns out our abilities were pretty different, so the training part never really materialized. So here are a few things I learned:
1. You may be able to work out together, but I wouldn't count on it. Riding a tandem can be a lot of fun, but you will have to be very flexible with your expectations about the training effect. I'll bet though that you spend a lot of time on the tandem just because it can be so much fun.
2. When you buy one, buy a decent one. After having a cheap one, then a decent one, I can definitely say that if I would have judged the experience only on the cheap one, we never would have stayed with it. Or as others have said, rent one for awhile.
3. Read this article before you ever get on one.
http://www.sheldonbrown.com/tandem.html It helped me more than anything.
Hope this helps.
Jeff