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Roy, this is definitely a complex task--which should take careful planning and organization. The steps that are outlined in the link that you provided is a good start, but that's only a small part of what needs to be done .There are a lot of sub-tasks that need to be taken care of in between those larger tasks. When it comes to moving site A to B, there is no site C involved--so just think about it as if you're moving site A to C and then B to C. Or, you could also first think about combining both sites and rather than moving site A to B you can choose the best content on each and then just move them to site C. What's important, though, is to figure out which content and pages are duplicated on both sites and then choose the best page(s) and move those to site C. There will be content that's essentially not on both sites, so those can just be moved. The key is to spend plenty of time organizing the content and deciding which content can go away, which needs to be moved, which needs to be combined, and soforth. There is one major step that's missing in that other list, which is to use verify all sites (http and https, as well as http://www and https://www) in Google Search Console, set up those 301 redirects, and use the Google Change of Address tool to tell Google that the site's moved. There is also a mention of rel canonical, and since the sites are moving entirely, canonical tags won't be appropriate to use. You'll need to use 301 Permanend Redirects to move the content from one site to another, especially since site A and B won't exist anymore (they'll be redirected).