This is the simplest way I have found to patch multiple installs.

I run 2 wow installs. 1 for the main and 1 for the 4 alts.

My dirs are.

D:\wow1
D:\wow2

Step 1: Run your first intalled copy and update. (It will Dl the patch and install.)
Step 2: Rename first directory to something esle. IE: D:\wow1 changed to D:\wow1temp
Step 3: Rename second directory to mirror the original. IE D:\wow2 changed to D:\wow1
Step 4: Run the second copy of the game and itll auto patch just like the first
Step 5: Rename the directories to their original names.

Voila. Simple and effective.