I've had enough problems getting my single bike flown around, I'm 6'7" and my single(s) won't fit in a standard bike box without disassembling the fork from the bike and removing all the cabling from the handlebars. Even then the box is bulging.
Not to mention the uneven application of airline policies, as noted by some others. Have the conditions of carriage and airline policies printed out with the applicable portions highlighted.
I don't think I'll be flying my tandem anywhere, sorry to say. I'd rather just ship two singles.