I used AHK this summer to box a browser game, and what I did was rename the window to the name of my toon.

My code to create the 3 windows:

Code:
MakeWindowA()
    {
Run, C:\Program Files (x86)\Mozilla Firefox\firefox.exe -new-window "http://www.nodiatis.com"
WinWaitActive, Nodiatis - Free Online Multiplayer RPG
WinSetTitle, Zenga
WinMove,,,0,0,780,780
    }
MakeWindowB()
    {
Run, C:\Program Files (x86)\Mozilla Firefox\firefox.exe -new-window "http://www.nodiatis.com"
WinWaitActive, Nodiatis - Free Online Multiplayer RPG
WinSetTitle, Vasco
WinMove,,,-780,0,780,780
    }
MakeWindowC()
    {
Run, C:\Program Files (x86)\Mozilla Firefox\firefox.exe -new-window "http://www.nodiatis.com"
WinWaitActive, Nodiatis - Free Online Multiplayer RPG
WinSetTitle, Ugolin
WinMove,,,781,0,780,780
    }
+1::
MakeWindowA()
MakeWindowB()
MakeWindowC()
return
Then for instance to move left I used

MoveLeft()
{
IfWinExist Vasco
{
WinActivate
Random, rand, 300, 500
Sleep, %rand%
MouseClick, left, 300, 375
}
IfWinExist Zenga
{
WinActivate
Random, rand, 300, 500
Sleep, %rand%
MouseClick, left, 300, 375
}
IfWinExist Ugolin
{
WinActivate
Random, rand, 300, 500
Sleep, %rand%
MouseClick, left, 300, 375
}
}
a::
MoveLeft()
In my example I generate a fixed mouseclick in every client after I hit A on my keyboard, which is against the WoW ToS, but it should be fairly straightforward to just send a key to each window. As you can see I wrote out every step rather than making 1 function, which is obviously not the best coding practice.

I'm by no means experienced in AHK, just needed a quick and dirty little tool. But if you want to look into a more advanced keybroadcaster in AHK you can head over to the consortium forum, the home of all the wow players who like to make gold. They have a tool called the Consortium Key Sender (CKS), and the code comes along with the an exe, so you can review it yourself. http://stormspire.net/content/ then click tools and cks.

But if following the advice to go to Hotkeynet is probably the best one you can get.