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.
This is typical for tandem axle trucks. Two axles are live axles, and the driveshaft goes into a power divider. The power divider supplies power to both axles.
They both do. The driveshaft turns a differential which distributes power to both axles.
Tandem by definition is two. So tandem axle is two axles. Example would be a tractor trailer having two axles next to each other is a tandem axle.
They both are. The driveshaft goes to a differential (the power divider), which supplies power equally to both axles.
A tandem truck is a truck with two drive axles.
It has two drive axles.
2 you fool
Five
There are two configurations for a tandem truck... the first - more common in Europe - is known as "single screw". The driveshaft goes to the differential of one axle, and that's the only live axle. In cases such as this, the additional axle is typically a lift axle. In N. America, tandem trucks normally use a twin screw configuration, where the driveshaft goes to a power divider... that power divider is essentially a differential, and has outputs going to both axles... ergo, both axles have power supplied to them at all times. A lot of people think that the power divider lock engages one of the axles.. this is not true.. it simply locks the outputs so that they always rotate at a matching speed.
A tri-axle has two live axles and a lift axle.. if it's located in front of the drive axles, it's known as a pusher... if it's located behind the drive axles, it's known as a tag axle. With a tandem axle setup (which is what tri-axles, quad axles, etc. are, albeit with the addition of dead axles), the driveshaft runs from the transmission output to the power divider. The power divider is a differential which transmits power evenly to both live axles.
Three - the two drive axles, and the steer axle.
A dump truck with two drive axles.