Why would you want the lights on 12/12? If you are trying to get the smaller one to flower earlier it won't work. Cannabis can only flower once mature, which is why a seedling can't flower.
I already have one autoflowering plant in its 9th week ( 5th week of flowering) my questions is would it be okay to start to grow another autoflowering plant in the same closet and start it out from the beginning on 12/ 12?
Germinate, sprout, wait a few days then give it light. Some varieties say 24 hr. others 16-18.
I can't imagine what the benefit of having less light hours for autoflowers. They flower based on their age, not light hours. All you would be doing is reducing your yield. What variety for example says 16 hours? Most that I've seen say 24/0 or 20/4. I haven't actually seen any that say 18/6 on the tude. Not that it really matters, regardless of strain the ruderalis species of cannabis doesn't care about light hours. 12/12 really only works on photoperiod strains.
195w CFLs, Northern Lights and Blue Mammoth autos - 70g dried! http://www.rollitup.org/grow-journals/525136-northern-lights-x-bb-auto.htm
Yeah switch your timer to a longer cycle and yo will have no problems doing another auto next to it. As the guys are saying, your current auto has no reason to be restricted to 12/12 anyway.
"Today is victory over yourself of yesterday, tomorrow is victory over lesser men."
you said it yourself. it is an autoflower. it's got nothing to do with light duration.