Two single cardboard boxes (if you don't have a tandem box), rear derraileur off (just the bolt, not the cable) and tied out of the way to the frame, on the wheel droppout mounts put those separators that replace the hub and keep them from being crushed, pedals, handlebars and seats come off, cranks can stay on, you might want to have two separate boxes, one for the frame and another for all the loose parts, but you can put them all into the same box as long as you tie everything together well and protect the frame from stratching). Lube the pedal threads or take lube for when you mount them. Be sweet at check-in, dress for adventure looking like you are going for an around the world tour with sponsors and offer to put the airline's logo on your web page is they sponsor you by not charging $ 100 for oversize.
We now have an S&S coupled bike, but I am now thinking that the time involved in packing and unpacking the tandem into cases might not justify the $ 100 oversize unless you travel a lot and will have trouble with local transport to and from the airport with a tandem box (you can always pedal to and from the airport, but I'm not sure if I'd like to do it in Bangkok). If I were to do it again I think I might have kept a lower cost tandem around for travel and just box it, the S&S premium plus the cases can go a long way towards second bike.
.... have trouble with local transport to and from the airport with a tandem box .....
For us, that is the deal braker.
We have an S&S triplet that we can also take as a Tandem. We fly with it at least once a year. Takes about 90 min to go from a triplet to a tandem in the cases and like 60 min from the cases to to road as a tandem and viceversa.
All good advice!
Also, suggest going to local home improvement store and buy sections of various diameter foam pipe insulation/tubing; cut it to size to fit over tandem frame tubes . . . it'll help save paintjob even inside the box/case.
Enjoy some of that spicy Thai food and some good tandeming TWOgether!
Rudy and Kay/zonatandem