A small program without a UI that will broadcast keystrokes from one WoW to the others.
Instructions:
Open all the WoW's you wish to use.
(Note, the first WoW opened will not have keys send to it)
Open WowMultiInstancer, keys 1 through = will (1,2,3,4,5,6,7,8,9,0,-,=) be sent to up to three other WoW processes.
Yes, I know it is basic, but with macros and maybe some new key bindings, it works perfectly for me.
This program was made in AutoHotKey. MultiInstancing will not get you banned.
Download: http://www.mediafire.com/?ijojy2mzftz
Virus scan: Virustotal. MD5: dac9010dff5ae6f41c5120f31e0f4a7e Heuristic.BehavesLike.Win32.ModifiedUPX.C!87 Trojan.Vilsel!IK Trojan/Win32.Vilsel.gen
Alright, the .exe comes up with a few questionable things, so I have uploaded the source.
Source: MultiBox.ahk
Edit: Just wanted to point out that the code is mostly code distributed across MANY various websites, including WowWiki, so I can't take credit for that part. I only put it together for the keys used.