The usual setup for a tandem synch chain has the two cyclists pedaling in exact synchronization, to when the captain's right pedal is straight up the stoker's right pedal will also be straight up. This is called "in phase."
If the cranks are at different angles from one another, they are described as "out of phase" sometimes with an angular measurment. For instance, if the captain's right pedal is at the top of the stroke while the stoker's right pedal is at the bottom of the stroke, that would be 180 degrees out of phase. (Nobody uses 180 degrees out of phase, it just makes the bike handle weird with no benefit.)
if the captain's cranks are horizontal when the stoker's cranks are vertical, that would be 90 degrees out of phase.
A minority of tandemists prefer "out of phase" setups, usually 90 degrees, with the captain's cranks 90 degrees forward of the stoker's. The advantage touted for this system is that it smooths out the power curve so that one of the riders is able to exert full force on one of the pedals at all times. Some folks believe this offers benefits in climbing, but I don't know of any evidence to support that theory.
90 degrees out-of-phase setups also reduce maximum stresses on the frame and drivetrain, so parts mighg last a bit longer this way.
There are a couple of serious drawbacks to this system:
* The riders must exercise great care in cornering to avoid striking a pedal. * The riders body movements are out-of-synch, which reduces the feeling of "togetherness" and also can complicate steering/handling especially at slow speeds.
Some tandemists prefer a setup where the cranks are only slightly out of phase, typically with the captiain's cranks just a few degrees ahead of the stoker's cranks.
Fortunately, no hardware changes are required if you wish to experiment with out-of-phase cranks. It's just a matter of removing and re-installing the chain.
There are essentially three entities riding a Tandem: The captain, the stoker, and the spirit. It is the spirit who likes in-phase cranks.
As I say, I've tried it and found it had some advantages. I thought of making up a Pattent Devvis so that you could start off in phase, then flip the 90 degrees once you were under way. Coulkd have made my fortune if both the tandemists in Britain had bought it.