tags-table-list

tags-table-list is a variable defined in `etags.el'.
Its value is

nil

Documentation:
List of file names of tags tables to search.
An element that is a directory means the file "TAGS" in that directory.
To switch to a new list of tags tables, setting this variable is sufficient.
If you set this variable, do not also set `tags-file-name'.
Use the `etags' program to make a tags table file.

You can customize this variable.