I saw a really large used Cannondale on CraigsList a few months ago. This would probably be the best bet for you. Remember, any really large steel bike may end up feeling like a wet noodle under two big riders, especially a six foot stoker. My five foot stoker with me at 5'9" made a quality steel Stantana feel pretty sub-optimal flex-wise. I didn't realize how good we had it on a Cannondale. The advantage you have is that most tandem teams are looking for medium to smaller frames in back. So the large ones don't sell very fast. But there are fewer out there for sale. Good luck searching.