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
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?