I utilize most of my off sites to link back in to my main site that drives my checkbook.
Now the key is the sites are not duplicates. As LadyH said, I would never create a site about about sales and then one about truck sales. I would have an automobile sales site and then one on automobile repair. Now the money maker would have “0” outgoing links and the other would link back to it.
All of my link exchanges and marketing would focus on the first the second would be bait.
If the sites are dups then you are also going to tread a fineline with the search engines so I would not go there either.
Just my two cents.