No, stays reserved at the same campground for the same dates will not automatically be next to each other. You can utilize select-my-site for an additional charge to choose your sites, or you can add a message to the campground during checkout to ask for site placements which will be at the discretion of the campground manager.
That doesn't mean this area has to always be empty. When you start reviewing camping options, your history will display here to help compare sites and find the best stay. You will be able to share your stay information with friends or family and save it for a later time if you have a KOA Account.