Well, since you flipped the tire around and it wore on the other side, it is unlikely that it is the tire itself.
Therefore, something is making that side of the tire spend more time in contact with the road.
It could be a camber problem. Make sure that with the frame perfectly vertical, the front tire is perfectly vertical as well. If it were crookedly mounted in the fork, or if the fork is bent, that could be a problem.
It could be you ride the bike crookedly. Get someone to watch you riding and see if you lean the bike over consistently to one side. That seems unlikely especially on a tandem, plus I'd think that would result in both tires wearing on one side.
It could be you mostly turn in the same direction. Do you have a "normal" route you ride? Check it out, are almost all the turns the same direction?