When deciding on the size of our tandem, I did not consider the reach that my stoker would need, which was a mistake. She has a long torso, and so needs her bars to be right up under my seat. My seat is therefore a lot further forward than I would normally have it to accommodate this. To get my reach right, I've now had to go to a 130mm stem, whereas I might normally use a 110mm stem given the top-tube length.
I never realized that the two rider positions would interact like this until we started playing around. Next time (if there is one), I think we'll go for a custom build to get the fit right for us both more easily. Simply getting the next size frame would not have been better - it would have been too tall and so would have caused other problems.