You can select the library you want to use for each project. Each symbol library set must be in its own subdirectory, but adhere to the AutoCAD Electrical toolset file naming convention. You cannot have duplicate symbols in the various symbol libraries.
To set a symbol library to use for a particular project, right-click the project name inside the Project Manager, and select Properties. In the Project Properties 
Project Settings dialog box, click the plus sign (+) next to Schematic Libraries or Panel Footprint Libraries. Click Add and enter the path of the library into the edit box or click Default to use the default libraries.
You can include electrical, pneumatic, or other schematic libraries in the path.
You can also include a series of library paths for AutoCAD Electrical toolset to use. Enter the names of the libraries (in order) with a semicolon between them. For example:
C:/Documents and Settings/All Users/Documents/Autodesk/Acade {version}/Libs/;C:/{user path}/{user library}.
AutoCAD Electrical Toolset search sequence
AutoCAD Electrical toolset runs through specific search sequences when looking for your symbols.
- Looks on the drawing for a copy of the requested symbol.
- Checks for the specific file name if a full path name is provided.
- Checks in your user subdirectory (given by the WD_USER setting in the .env file).
- Checks in the directory where the active project's .wdp file is located.
- Checks in the selected library -- it is the library selected per the active project.
- Checks the directory containing AutoCAD Electrical toolset support files.
- Checks the current directory.
- Checks the path given by the AutoCAD Electrical toolset environment variable.