Here is an idea:
Have two copies of WoW. So for example one at c:\WoW1\ and one at c:\WoW2\. You can just copy your old wow directory and rename to WoW2, you don’t have to install and patch the game if you want more copies. Now let’s say you use WoW1 on the widescreen monitor and WoW2 on the old one. Go into c:\WoW2\WTF\Config.wtf open it with some text editor (notepad etc…) search for the line starting with “SET gxResolution” and then just replace the resolution with whatever resolution you want to run your 2nd WoW with on the older monitor. There are probably restrictions on what kind of resolutions are allowed else it sets back to default, but going with like 1280*1024 or 1024*768 will surely work.