At first we did not think we would be able to transport the tandem bike to France and were thinking of renting bikes there. I called US Air at their national number and was put on hold for a long time. I told them that the bike box would be 100 inches long. This was with the wheels off and the front fork turned in, derailleur off. Finally they came back and said they would take it. I then called our local airport and asked them. I was also on hold for a long time. They apparently looked up our reservation, noticed that we had a leg on Lufthansa, and checked with them too. They also said yes. So we started packing it!
We took all the major removable items off the frame, including the seats and wheels. We handlebars are attached to the front seat mount on this bike, so we were able to turn the handlebar parallel with the frame.
We found the frame would just fit in two bicycle shipping boxes (very nicely given to us by Cayuga Ski and Cyclery in Ithaca). So we put them together with pop rivets and big washers. I cut some pieces of plywood and put them together with dowels to protect the front forks and bottom bracket. These structures also helped keep the box together some. They were screwed in from the outside.
We put the remaining large parts in another bike box - the seats, wheels and some other small parts. We put some random pieces of styrofoam in both boxes, but I'm not sure if that helped.
Both boxes were secured with duct tape here and there and auto roof-rack straps. We tried not to close the box with duct tape so we could let customs and airline people see what was inside just by loosening the straps.
We had no trouble with the boxes, but the big one was pretty much of a wreck when we got back.
The airlines never objected to taking them.

Packing and unpacking the bike