Here is a simple "hello world" for hotkeynet:

First download the hotkeynet app:

http://hotkeynet.com/

Next start 2 instances of wow

Then start hotkeynet and and relabel the wow windows through the hotkeynet menu, for this example call your primary wow instance wow1 and the second wow instance wow2

Load this script:

Code:
<Hotkey 1>
<SendWinM wow1>
<Key 1>
<SendWinM wow2>
<Key 1>
Set up a macro or something on each character bound to key 1, hit 1 while your in the wow1 instance to confirm that it works and the key is sent to both windows. If everything is working then you can expand the script from there, this thread has excellent tutorials for getting started

[HotKeyNet] Guide: HowTo use HotKeyNet for boxing

hth