Have you considered using a frame bag? On many tandem frames you can put a frame bag attached to the stoker's top tube, and captains' seat tube that is not in either riders way.
A moderate size frame bag, and a decent sized saddle bag would likely be more than adequate to carry the amount of stuff you're describing.
Easier, cheaper, and lighter than a rack and panniers, or trunk, and carries the weight lower than a rack and trunk combination.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.