The LIBNAME statement when given two or more locations as its library specifications has a default behavior generally described as the library specification becomes a "search path" for individual members in the concatenated library. A LIBNAME that instead choose, when it is necessary, the youngest member of the libraries by their modified dates as its primary target file.
Rules for Library Concatenation (For this new feature)
After you create a library concatenation, you can specify the libref in any context that accepts a simple (non-concatenated) libref. These rules determine how SAS files (that is, members of SAS libraries) are located among the concatenated libraries:
When a SAS file is opened for input or update, the concatenated libraries are searched and the firstthe youngest occurrence of the specified file, by modified date, is used.
When you delete or rename a SAS file, only the firstthe youngest occurrence of the file is affected.
Anytime a list of SAS files is displayed, only one occurrence of a filename is shown.
Note: Even if the name occurs multiple times in the concatenation, only the first occurrence is shown. Information pertaining to the youngest occurrence is displayed if queried.
5 - 8. (same)
However this would be implemented; by an option to the default v9 engine or perhaps as an separate, alternate engine by itself.