MTB shoes are a personal preference item. My wife uses double-sided SPDs so she doesn't have to think about clipping in. I use A520s since with the daVinci I've got a bit of latitude in getting clipped in. On a conventional tandem I'd definitely use the double-sided pedals.

We have a ClickStand which we use on large organized rides; often there is no place to "park" a tandem at the rest stops - your basic novelty item.

And the usual stuff: Old Man Mountain rack, Arkel trunk, Lezyne Micro Floor Drive HP pump, Garmin 800, Thudbuster. I call out "bump" on my single, but it's impossible to catch every bump on the tandem and the stoker pays the penalty. Depends on seatpost, stoker weight/age, etc. If your tandem has a disc rear brake I can really recommend the Shimano Ice XT rotors; Avid also has a rotor with an Aluminum spider, but not the sandwich rotor of the Shimano rotor.