Well thats a bit tricker then simply migrating one site to a new domain…

So what I would do is use an SFTP client and download the media from site 1 and then migrate it over to site 2. This is assuming the media is completely different and there won’t be any naming collisions (files with the same name in your uploads folder).

Now use the export tool to export the posts (and don’t include the media this time – you took care of this in the previous step) from site 1 and import that into your site 2.

You should now have the posts and associated content from site 1 merged into site 2 without overwriting anything already in site 2.

Anyway, hope this helps!