Is it possible to fly a tandem-rotor helicopter after a single rotor failure?
It's been a few decades since I crewed CH-46s, but the situation you're describing is  called a de-sync and it's a catastrophic failure. The forward and aft rotors are linked by a driveshaft that drives the forward transmission and synchronizes the rotors. In a de-sync the intermeshed rotors will collide and the aircraft will tear itself to pieces.

Also, setting aside for a moment the consequences of a de-sync, the flight controls for both rotors are linked so it would be impossible to flatten the pitch to autorotate just the front rotor head. I suppose it would be mathematically possible to keep the rotors in sync and match the rate of deterioration in the rotor speed, but the odds would be extremely low, and the chances of recovering from any altitude beyond a few feet would be near nil.