[Divingduck]

An other workaround can be to move all books (book folder) in an new library folder and then open the new library. Doing it that way results in a library with an incomplete database file metadata.db because it will not match to both libraries (or in an empty database if you forgot to copy that file). The book folders itself have all necessary information's to rebuild the metadata.db. Run ->library maintenance ->restore library for rebuilding the library based on all existing books. Afterwards you need to check the library once more and correct the findings.

Note: This is not a general way to consolidate libraries (there are better solutions) but a workaround if all other way's won't work as good solution for a special case.