I've taken two tandems to France and brought one back. The first one was about 15 years ago when Air France was still taking bikes for free. I just told them it was a bike, and they didn't charge me. That tandem is still in France (and will be for sale if anybody would be looking for a used Cannondale)
The second time, there and back, was two years ago. It was on Condor Air, which I think is some offshoot of Lufthansa and got me to Toulouse through Frankfurt. I bought business class tix, which was only around $2k, and they again took the tandem free each way.
In both cases, I basically got the big box as light as possible, well under the normal max weight, by packing the wheels and many of the parts in a separate box. I'm probably going to take another one over in the next year or two and will likely follow a similar strategy.