Here is how I set up to run both FHLC versions on the network. There may be a more elegant way but this worked so I stopped looking.

You may need to adjust the drive letters in your actual setup. This assumes that you have Family search already installed and working for FHLC Version 18.

1. Copy contents of FHLC Version 17 to a directory on the server (c:\lc17) Share this directory read-only.
2. On all systems that you want to be able to use both FHLCs
3. Map the lc17 directory to a network drive (you can use subst on the server if it also runs Family Search) Assume it is mapped to "O"
4. In the \pas directory copy the lc directory to lc17
   You can use the command "xcopy /s lc lc17" from the \pas directory
5. Copy the file pasetup.txt to pasetup1.txt
   You can use the command "copy pasetup.txt pasetup1.txt"
6. In the \pas\lc17 directory edit the file apsetup.txt and change the line that reads
   PASETUP=C:/pas/pasetup.txt to read PASETUP=C:/pas/pasetup1.txt
7. In the \pas directory edit the file pasetup1.txt and change the line that reads (the letter K is where you have the Version 18 FHLC mapped and the letter O is where Version 17 is mapped)
   LC.EXE=K:P
   to read
   LC.EXE=O:P
   
8. Add the following line after the similar lines in the file
   DRIVEO=CD-ROM (Network)
9. To add the Version 17 Catalog to the main menu, in the \pas directory, edit the file menfile.txt Find the line that begins
   ITEM, @MEMBER_1@, @Family History Library Catalog@,
   Copy this line and the next 2 lines placing them directly below the original lines. In the copy change
   Family History Library Catalog
   to read
   FHLC (17th Edition)
   In the next line (which begins with RUN_PROGRAM) change
   c:\pas\lc
   to read
   c:\pas\lc17