I'm looking at a vintage crank I'm interested in but it is listed as a tandem crank. How is that different from a regular crank?

If it's the Captain's crank it will have a spider on the left side for a single synchronizing chainring and no spider for any chainrings in the right or normal drive-side. Also, the crankarm with the spider will have left-handed pedal threads like a regular non-drive-side crankarm, which is why you can't just turn a standard crank around 180° to get the chainring on the left. The pedal threading would be wrong.

If it's the Stoker's crank it will have a both a spider for the synchronizing chainring on the left side and a regular spider (usually for a triple) for chainrings on the normal drive-side.

A Captain's bottom bracket will have to be long enough on the left to accomidate the synchronizing ring and the Stoker's bb will have to be long enough to allow chainrings on both sides.

A tandem has two pairs of cranks, and a synch chain running between them. A front/captain tandem crank will have a chainring spindle for the synch chain on the left arm, while the rear/stoker crank will have chainring spindles on both crank arms.
It's not just a question of flipping them over, as arm orientation is coordinated with pedal threading. Get it wrong and pedals may unscrew while JRA.