We’ve flown with EasyJet and Jet2 in Europe without any problems we used to pack the tandem (a Dawes Discovery Twin, no couplings) in a big bag padded with cardboard and was probably no longer than a surfboard once the wheels and racks were removed and placed alongside the frame, no one has questioned the size at check in, we just tell them it’s a bike.
More recently we’ve been packing the tandem in cardboard bike boxes from a bike shop, it takes one full box + about 12 inches of a second box taped to the end to fit it in (with wheels and racks remove as above). We did have a big green bag to put the box in, but have since lost this and just wrap the box in cling wrap.
After bending a chainring on one flight I now remove these and the disk brake rotors and bundle them al together safely.
If you search the discussion board for ‘flying’ there’s a whole load of posts with useful information.