How to mount clipless pedals on a tandem? My wife and I have recently purchased a vintage tandem, and are willing to switch to Shimano SPD pedals (M520).

The problem is that there are 3 "chainring" sides on a tandem, 2 of which are located on the left side of the bike. So basically I don't see any way to fit the pedals in the correct position, except for the stoker right side, because the threads will be the wrong way round.

How do I fit normal pedals onto tandem cranks?
Tandem cranks are special. The pedal thread directions on each arm are oriented correctly to prevent loosening from precession, as on any other bike. You just need two sets of normal pedals.

If somehow non-tandem cranks have been installed such that either of the left cranks are right-threaded or the right captain crank is left-threaded, that situation is not acceptable and threadlockers will not make it so given the danger level of the crash that could result. Either get tandem cranks or get the offending crank(s) helicoiled to have proper thread orientation to resist loosening.