Why does my tandem axles each have a driveshaft?

Because both axles are powered. You have what's known as a 6x4 truck.. the driveshaft comes off the transmission tail shaft, and it goes into the interaxle differential pinion. That interaxle differential provides torque to both axles at all times - not just when the interaxle differential lock (sometimes referred to as a 'power divider lock') is engaged. Locking the interaxle differential simply locks it to ensure that outputs to both axles rotate at the same speed.