There is no iron clad way of merging two legato servers without the chance of great error. The reason is this: When a backup is sent to the server, the actual data is sent to tape while control data (info about the data) is written to the index and media database. These records on both databases are tied together by a unique identifying number. This number (ID) is unique to this machine only. If you merge databases from varying installations, you run the chance of duplicate ID references in the databases. This WILL cause corruption in the database.
This is a gamble my friend and though the other response is accurate in logistics, the concept is simple--corruption is not an option.
Hope this helps.
Matthew