Here the absolute easiest way to patch for users on Vista or Win 7.

Step 1) Run one wow let it patch

Step 2) Open the following folder:
C:\Users\Public\Documents\Blizzard Entertainment\World of Warcraft

Step 2.5) Sort by Date Modified
Find all files created on current patch date
Usually 2 files an mpq file and an exe.

Step 3) Copy said files to:
C:\Users\Public\Games\World of Warcraft aka root WoW folder for each client. This is the same place you could find each WoW.exe and Launcher.exe file.

Step 4) Run the downloaded and now freshly copied .exe file you just put in the folder.
Do this for each folder you have for WoW

Step 5) Play game

yup that's it it's really that simple. If you have files symlinked this still works just fine and simply means you have to copy the downloaded content fewer times. I personally run 5 completely separate folders for WoW and have never had this fail on me. The reason you can't run the update normally for each one is a registry key that is exclusive and always tries to update only the last location WoW.exe was installed in or run from.