All libraries you wish to specify must be on the same line. Note the use of the "\" line continuation character at the end of the first line, which tells the interpreter to concatenate the next line with the previous one.
You can also do this in command line mode using the -y option as follows:
Say you have libraries in directories lib1 and lib2, and the extensions in these libraries are ".v" and ".vpd" respectively. To specify the two libraries on the Verilog command line, you would invoke Verilog-XL in the following manner: