Thorn make them with S&S couplings and you can find 2nd hand ones available. They breakdown to leave two sections, each being smaller than a single bike. Hope this helps.

We have an Orbit that breaks down with SS couplings from JD Tandems they break into either 2 or 3 pieces and are built to your spec

We purchased a Santana eSCape tandem about 4 years ago which breaks down to fit in Santana's own SafeCase. It's a beautiful piece of kit, and the tandem fully built up weighs just 13kgs. In the SafeCase it has an all up weight of 30kg, which is within the 32kgs most airlines allow.  The only thing is the cost, but if you're planning on doing a reasonable amount of flying it's one of the best ways to do it.  Check out https://santanatandem.com/Bikes/InStock.html.  

We have a Thorn Twin Raven with S&S couplers which are fantastic.The bike fits easily into the back of an estate car and we have consequently used the bike far more than if we couldn't transport it like this. Also we can tow a caravan and take the bike. For us not specifying the couplers would have been a big mistake. Rigging the bike takes about 10 minutes and is very easy to do. Have fun.

I would recommend talking to Pete or Rob at The Tandem Shop. We considered getting couplers and these are excellent. https://www.tandeming.co.uk/brands/co-pilot-couplings/

We had S&S couplings on our last tandem. Worked perfectly. Old tennis balls with a X cut in them make good protection against knocks when couplings are not connected.

It's always worth checking with who you intend on flying with.  Lufthansa told us not to break the tandem down and make it look as much like a bicycle as possible, that way they would treat it more carefully (they charge the equivalent of 2 solos).  When I contacted Virgin to fly to the west coast of the USA they also recommended not breaking it down and to ensure we gave plenty of advance notice.  With Lufthansa we took off the pedals (we were asked not to turn the handlebars) and wrapped the bike in cardboard leaving the bottom of the wheels exposed so it could be easily pushed.  When we got to Munich it was simply a case of throwing the cardboard away at the airport and then starting the journey.

When I've travelled with my solo bike in the past, I've only ever used the large see through plastic bags that CTC recommend - link below is them being sold via wiggle.

https://www.wiggle.co.uk/ctc-cycling-uk-plastic-bike-bag

I've never had any issues with travel like this, but different operators have different ideas of what is needed to be done, even within the same company, so sometimes I've had tor remove pedals, wheels, deflate tyres and rotate bars but sometimes nothing at all. Best thing is to be prepared to do all of those things if asked and allow plenty of time at check in. I don't think its possible to get a tandem in one of these plastic bags though. If breaking a tandem into two pieces then you would need to tape the two section side by side with strong tape to stop them moving against each other

• Hi Warren, we have a Thorn Raven with split frame, and lots of extras which we're selling if your interested. We also have a Bobyak trailer, waterproof bag and 4 Ortlieb panniers. Happy to discuss price if you're interested. Kind regards Bill 

Hi Warren, I once flew from Gatwick to Nantes on a small Air France regional jet with an Orbit tandem. I asked before flying about preparation; I was asked how big the tandem would be when made as small as possible. I worked out that with wheels and mudguards and racks off and handlebars folded round it would be about 6ft 6 x 2ft x 9 inches, and I was told that that was OK because it was about the same size as a coffin and they could carry those. My friends went on a BA Airbus and had to do no prep at all, but on their return flight had to remove pedals and deflate tyres.

I've also dumped cardboard at airports on arrival and gone to a bike shop at the end of a trip to get boxes to pack the bike to fly home.

But it's a real nuisance dismantling the tandem to the extent required sometimes; I'd get a tandem with S&S couplers given the chance.