Many people propose hybrids in which the engine drives one axle and an electric motor drives the other, than there's no integration of the two drive systems. None of them are actually built because none make any sense, because there is too much weight and complexity for too little benefit... but maybe you'll be the first. 
There's no need to mechanically disengage the electric motor. If the motor is going to spin without power most of the time, an induction motor is likely more suitable than a permanent magnet motor.
There's no need to mechanically disengage the electric motor. If the motor is going to spin without power most of the time, an induction motor is likely more suitable than a permanent magnet motor.