Maybe I am missing something, but do you even have a tandem yet?

Some of the questions you are asking are a bit premature. It sounds like you need a lot more two-gether saddle time before you broach the subject of marathon butt bustin'.

How do you do on a ten mile ride on a tandem? A 50 mile tandem jaunt?

The answer to your question is... If the two of you believe you can ride 150 miles in one day on a tandem, and you two are both willing to work towards that goal, and you have some time in which to do this training, I am sure you will be able to.

Slow down, get comfortable with your stoker. You might decide yourself that you don't want to go through with the tandem ride after you ride together for a while. Remember, being on a tandem is about a shared experience. Make sure that shared experience is enjoyable, not drudgery.