I'd guess, the manual controller would try to close the outflow valves while the pilots did the heavy lifting of STARTING THE APU,
As far as I know, manual is only controlled by the pilots using battery power (no main bus DC power available). During a dual engine failure (prior to APU start), the outflow valves will remain in their last position.
If the outflow valves (in the frozen position) happened to be letting out more air than required then the cabin will start to depressurise. Since there will be no power to the "auto" motors, I would say that the 14000' backup function of the "auto" motors would not work. There is no 14000' backup function in the "manual" motor.