So, we have 2 different sites, both on the same version of 11.x. We would like to make a multi-site out of them. So, we merge the codebase from both under one solution. Now, there is the database part that needs to be merged into one - whats the easiest process for content migration from database B back to database A so the end result has both sites content in it?
If it's just CMS you can use the import/export tool. It does struggle with larger datasets but you can break it in to smaller sections.
Alternatively you could build a scheduled job that use the content mangement API to read the information and then programtically recreate it.
You'll likely need some sort of content freeze when you plan this