S & S couplings for us.
I much prefer not to split the tandem, because where you have to change trains, it becomes a right PITA.
We've put the tandem in one piece in GWR guards' vans, Eastcoast (and whatever their forerunner was), East Anglia big trains and the shuttle services between Southend & Lpoo & Fench St, Trans-pennine express.
The trains where we've had to split it, and had no problems, were south-west trains to Portsmouth / New Forest, and the guard there was actually familiar with S & S couplings. Without S & S we'd have been blocking the door and wouldn't have been allowed on. For the sleeper, with dangly bike spaces, it needed to be split, and on Virgin Supervoyagers. On the sleeper, we had to get up at 1 a.m. in Edinburgh because when several trains join together, some guards vans get taken out and we had to wheel the two halves of the tandem about 500 yards along a platform. I've never seen such a long train!
The only real issue I've had with staff was at Fort William last April. I was on crutches and although we had booked bike spaces for 4 bikes, one of them was our tandem. Shouldn't have been a problem and fitted in the dangly spaces OK but a jobsworth decided to make her presence felt and was threatening to chuck us off the train. Her parting shot was "No more tandems, OK?" so I wrote to the Scots Dept of Tourism for clarification. Why no S & S tandems if you've got bike reservations? Their reply was that, if the tandem is split, they no longer consider it to be a bicycle and it's therefore luggage and it's up to the guard's discretion whether they take it or not. Sounds like bullshit to me.