Is autorotation possible in a tandem rotored helicopter?
On the Chinook, the sprag clutches that allow the drive system to freewheel when an engine fails are between each engine's step-down transmission and the "combining transmission" that receives torque from each engine and sends it to the front and rear rotor transmissions.

This means that the entire drive line between the rotors always remains engaged and in sync regardless of whether any power is being supplied. If one engine fails, it's simply disconnected from the drive system by its sprag clutch with the remaining engine doing the work, like any helicopter.  And if both engines are shut down, same deal; both engines are disconnected by their sprag clutches and the still interconnected rotor system will autorotate with the correct blade intermesh maintained.

So the Chinook can loose one engine and continue flying (more or less), or lose both both engines and glide to land like any multi-engine helicopter.  If you loose one of the gearboxes or drive shafts in the interconnected system however, now you have an unpleasant situation.