We've got an adjustable stem for our tandem. I needed more than -17 degrees to get the time trial position set up, and it works for that. Neither stiffness, nor loosening of the bolt is a problem.
But it is ugly, and a boat anchor. Personally, I'd go with a fixed stem once you have your position dialed in.
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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.