Here's a video on packing your tandem with 2 regular bike boxes. They just tape it heavily. I've put 2 boxes together with zip ties which has worked well. Just poke a couple of holes through both boxes when you have them lined up and run a zip tie through them. Of course you still want to bubble wrap or some other wrap the frame in the box, but if you are just shipping a frame and not a bunch of parts there is not much there to ruin your paint job.