AutoHotKey is an application that allows you to send a keystroke to more than 1 client on the same PC. It is scriptable and open source. There is another application called Keyclone that provides similar functionality but without the scripting. If AHK does not work for you and you do not want to go pure hardware, take a look at Keyclone.

http://www.autohotkey.com/

For those planning on dual-boxing with just 1 PC this is the application for you. Being able to push keystrokes on 1 screen and it sending the same to the other greatly enhances the effectiveness.
Although it does take a bit to set up, this is an example of setting up the "2" key to be sent to both World of Warcraft clients.
Code:
WinGet, wowid, List, World of Warcraft

~2::
KeyWait 2
IfWinActive, World of Warcraft
{
ControlSend,, 2, ahk_id %wowid1%
ControlSend,, 2, ahk_id %wowid2%
Return
}

Learning Curve - Medium - High

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