Hello,
The reason why you need both generators in the B737 for performing an autoland, is when you want to execute an CATIII approach you need both autopilot systems A & B switched ON. Each autopilot system is coupled with it's own electrical circuit, so they are seperated from each other and not fed by only one generator. Normally when you aren't able to perform an autoland anymore with the B737 you gonna switch to an fail-passive automatic landing system.