There currently isn't a way to merge two separate tsbak files from separate servers. But I think I have an idea that would work to get all the content onto one Tableau Server:
1. Restore the tsbak files to separate Tableau Server instances, running the same version of Tableau Server. We'll call them Server A and Server B.
2. Use the site export/import feature to move all of the sites from Server A to Server B.
3. Now, all of your content is on Server B - you can use this Tableau Server for your users, or take a tsbak from here that will now include all of your content.
Hope this helps!