There are no tools to migrate. Why not upgrade the site?
You could use the Kentico Import Toolkit (KIT) for version 12 to import those pages from the other instance. The problem is this won't import any of the attachments so those will have to be manually added.
Another option is to write some code using the API to get the documents from the v11 instance and add them in the v12 instance. This will allow you to get the attachment as well and add it in code without needing any manual intervention.