First importance is that captain can straddle bikel with legs far enough apart for pedals to move without causing injury. I suspect this is not a problem.
Check seat height adjustment - most likely not a problem.
If you are using drop bars on the tandem I would not use mountain bike sizing. Look at the captain's top tube length and see if you can get the captain's bars in the same position as your road bike (or a little closer/higher) by changing out the stem.
Ride it by yourself and see how it feels.
Good luck.