Tandems of the forum

I think with the loading you would be better going for less speeds and more robust chains and rings. Tandems are hard on their drive-train.

When you say say custom tandem, do you mean by a custom builder or from a dedicated tandem builder who does everything custom?

If you are buying from a dedicated tandem builder, they are almost certainly able to do custom geo for very little extra money (certainly our Co-Motion could have had custom geo for a little more money, but we fitted well on a standard frame sizing). Edit - just checked, it would have been about £350 extra.

'Tandem groupsets' are always a bit of a mix and match to make something that works. The backend of the drivetrain is pretty much a standard setup, though the choice of rear crankset is limited to the few companies that do a tandem setup (we have FSA Gossamer cranks and have had Middleburn before that).

Depends on what you are using the tandem for, but 11 speed begins to take you down a path that precludes a triple - fine for racing, but not so good for touring possibly. I have friends with an 11 speed Di2 setup that has a double - but that is on a zoom zoom racing tandem. I would guess they eat through rear chains fairly quickly. Many of the high end tandems out there are still being sold with 6700 Ultegra shifters and 10 speed rear dérailleur, FSA SL-K crank sets etc.).

Gates timing belts work well. We've not had any issues with ours at all (we have dual Carbon Drives).

Levers - if you are not shifting with them, then the standalone shimano ones are okay with road discs, but I find them a little uncomfortable and sharp, so need to swap them with a set of something with a wider hood that match the ones I have on the fixed (but I really can not remember what they are off the top of my head!).