Why is it called a "timing chain" Why is the chain connecting the front and rear cranks of a tandem called a timing chain.  This brings to mind big-block V8s and leaky timing chain covers that plagued my younger years.

What's the history of this term?
The Timing chain helps keep the front and back riders pedals rotating at the same time.  Depending on your preferences you can set them as "in phase" where both the front and back cranks are at the same point in the rotation, or as "out of phase" where the front and back cranks are offset by 90 degrees. This is advantageous as when one of the riders reaches the dead point (where the cranks are vertical), the other set of cranks is at horizontal allowing good power transfer.  You can set up different levels of out of phase. You can set it up so that one rider is only a few teeth ahead of the other rider, so that both riders aren't in a dead zone, but the pedals are more or less aligned.

You can even set up both riders with different size chain rings, which would allow one rider to pedal at a faster cadence than the other rider.  This presents problems when cornering, as it's hard to get the pedals into the proper position so that neither the front nor the back will hit the ground. 

Sources and more info:
Pedal Setup
Poll: In Phase vs. Out Of Phase
Out Of Phase Cranks