Sans- Thanks for clarifying my crappy description of the CH-47 rotor arrangement. I'm not an aero guy, but I believe the reason for locating the rear rotor on a pylon at a higher WL was to reduce the impact of downwash from the forward rotor on the rear rotor during forward flight or autorotation.
And an interconnection between the CH-47's two rotor drives to maintain phasing is indeed critical. The 1982 Mannheim crash you referenced is well known to rotorcraft drivetrain engineers. The root cause of the accident was loss of oil flow to the rear mast bearings, causing them to overheat and seize. After losing phasing the front/rear rotor blades impacted each other, and soon after the rear pylon/rotor separated from the airframe. The loss of oil flow to the rear rotor mast bearings was determined to be caused by debris clogging the bearing oil jet orifices. The debris was blast media (walnut shell) used to clean the gearbox housings during overhaul that was not completely removed from the oil galleries. As a result of this particular accident all rotorcraft transmission lube oil jets now typically incorporate a "last chance" debris filter.