The "what can possibly go wrong?" tandem: @YellowTim on the front, @McWobble stoking.
The "because you've got to co-operate" tandem:
Anyone at all piloting, @User as stoker.
The "because it would be amusing to see the tandem that would let them try" tandem: @User13710 as captain, @martint235 stoking.
The "their riding styles are so similar that they must be able to co-operate on a tandem just like they do in real life" tandem: @dellzeqq piloting, @Agent Hilda stoking.