Instead of cluttering up the article with long technical explainations about how the helicopter works, here's a separate window with that same information.

The diagram below illustrates the flow of power from each engine's power turbine to the rotors. 

CH-47 drivetrain layout.

Contrary to widely held opinion, one engine does not power one rotor. The rotor systems are driven by both engines thru a complex series of driveshafts and transmissions. Basically, each engine turns a small engine-mounted gearbox (called a "nose box"), which in turn feed a centrally mounted "combining transmission," or "C-box." (The C-box is marked "comb xmsn" in the diagram above. "xmsn" is shorthand for "transmission.")

The C-box combines the power and torque from each Lycoming (Now Allied-Signal) T55-L712 engine into one drive line. All 3,750 shaft horsepower per engine into one. That's a LOT of torque and power in anybody's book.  Notice the extreme reduction in RPM from the engines (15,066 RPM) to the rotor blades (225 RPM).  That's an overall reduction of 66.96:1! Wow.

So-called "synchronizing" driveshafts eminate from the fore and aft of the C-box to send the power to the forward and aft  transmissions. There are two driveshafts in the power train leading to the aft main transmission, and a chain of 7 shafts leading foward to the forward main transmission. Each driveshaft segment is separated by a flexible coupling (called a "flex-pack") and a hanger bearing, which provides a place to mount the shaft to the airframe. So for 7 shafts, there are six sets of hanger bearings and flex packs between the forward transmission and the combining transmission.

The structure of the aircraft is such that the thin-walled alumninmum driveshafts are very close to the upper structure of the fuselage. In a static condition, you  cannot push your pinky finger between the driveshaft and the top of the fuselage. These shafts turn at about 7,000 RPM, and the aircraft actually flexes longitiudinally (thus the need for the "flex-packs" between driveshaft) when it lifts heavy loads.

 

Diagram courtesy H Co., 4/7th Aviation Regiment

The fore and aft shaft sets are called synchonizing shafts for a reason: they serve to synchronize each rotor system, which allows the rotor blades from tehe fore and aft rotor systems to intermesh without striking one another. Were a sync shaft to fail, the rotor transmissions would no longer have a mechanical connection.  If that happened, as some of the crews would fatalisticly say,  "Its all over but the pieces hittin' the ground..." (In fact, in the emergency section of the copy of the aircraft checklist in the aircraft, the immediate action item for a driveline failure was "Land as soon as possible." You would normally find that crossed out with black marker and the words "Die immediately" replacing it.  Bright outlook, huh?)

You can see the result of the failure of a forward transmission input pinon on this page. Caution: This is a very, very graphic sequence of photos.  This helicopter accident is illustrated during the first week of the CH-47 maintainer course at Ft. Eustis, VA, when they talk about your responsibilities as a maintainer and the consequences of your work.  Talk about an eye-opener!