SEO best practice merging two sites together
After personal experience in trying to do this there are certain things you must consider when merging two websites content together. It isn’t a simple process of moving the content to one of the websites then closing the other website. Google remembers the source of the original content and would issue you with a duplicate content penalty. The best practice is to keep the old website online and use canocalisation on the old website directing Google to the new website.
Otherwise the other alternative to this is to have the old content rewritten.