I got a guy in Colorado to ship a Tandem for me. He packed the frame in two regular bike boxes joined together, the two wheels in two other boxes and a fourth box for all other parts. He took them to a Fedex shipping center and shipped them Fedex Ground, Funds on Delivery type transaction. It cost him $180 to ship the four boxes. Once I received them I handed over a Cashier's Check for the agreed price of the Tandem to the Fedex delivery guy. He received my check the next day. This way I paid only once I got all the pieces of the Tandem and he gets assurance that I pay up. This is my experience getting a Tandem shipped to me.
You could probably get the two wheels in one box and the frame and other parts in another to reduce shipping cost. This should be ok as long as the packed items do not rub against each other. Do not know how much this will save you though. Hope this helps.
I had mine shipped to me from Michigan. Professionally packed by a bike shop in Midlands MI, and I felt overcharged for a crappy job, but the damage wasn't too bad (bent derailleur, derailleur hanger, etc., all of which I was able to straighten by hand; but still, made me think: what did I pay for?). FedEx delivered the two boxes for $130, as I recall.
If you are close enough to a tandem bike shop, they will probably have a tandem sized cardboard shipping box and will be glad to give it to you. The tandem shipping containers are generally made of thicker, heavier cardboard than a single bike container. I suggest you wrap the bike tubing, forks, etc. in pipe insulation that you can get from Home Depot or Lowes. You can get it in different pipe sizes and it comes in 10' lengths. Use zipties to mount your derraileur to the inside of your chainstays, and definitely use dropout spacers to prevent your chainstays/dropouts from getting damaged. Once you have it wrapped and placed inside the box, fill the cavity with popcor packing material. I have done it both with FedEx and UPS. My choice is UPS. IMHO, they are better at handling large boxes. Take detailed before and after pictures of your bike so you have a record of what it looked like prior to shipping.