Not that so much as it doesn't max out the DLI (Day Light Interval). That's a theory that plants can only use a certain maximum amount of light per day and then they wait for the next day. BUT, most growers don't maximixe the light their plants can handle and therefore don't maximize the growth they could have had with more light.
One way to get more light is more hours of light, so by using a 20/4 for your auto's you have a better chance of maxing out the energy they can use for the day. But, photo's need something much less than 20 hours of light to trigger them into flowering. Most of us use 12/12 to be safe although the actual number is a bit less than 12 hours of darkness.
So, by flowering them together, at say 12/12, you are potentially shortchanging your auto's from the light they could have used had you made it available.