Yes, you can do that.
I have a recommendation for your Window Layout though that might provide you with a better solution, since you're 4 boxing Diablo 3 on 2 monitors.
Instead of having 1 big window on the main screen and 3 small windows on the second monitor, I use my second monitor for one big dxNothing
window, and I put a (fully interactive) view of each window using Video FX
in each of the 4 quadrants.
This way, my main screen broadcasts to all 4, but I can also just mouse over to the second monitor to interact with any individual
window through the Video FX, without changing the view on the main screen. So as far as looting goes I tap Alt in my main window so I can see all the blues and yellows and whatnot on my second screen, then I can just quickly move the mouse and click on them all as if they were one window (because at that point they are
). Additionally I can still use the main screen for any window the same way you use it, and I can make use of all the Video FX I want in all their full resolution glory (on my witch doctors that's pet health and mana balls next to the party health). My "Get more crappy loot" video
will give you a little peek at that, though I'll probably put up more videos about this particular Diablo 3 setup soon. (All the setup info to do this should be in the dxNothing and Video FX links.)
But as far as holding F2, F3 etc to send to a specific window, yes you can do that... the easiest way is to use the Mapped Key Wizard from the Wizards menu, and select "Broadcasting Toggler". If you tell it to use "pressed OR released" mode, it will toggle when you press the Hotkey, and then again when you release the Hotkey, which is the behavior you're wanting as far as "do this while I hold the key". Then it's just a matter of picking which window to send it to, and which window to broadcast from. It is important to note that this new toggle will not take into account the current state of Repeater (broadcasting); it is just expecting to turn on and off. It will restore the Repeater Target (where it broadcasts TO) to "all other" and turn off broadcasting. Since you'll probably normally play with broadcasting ON (and to "all other"), then after completing the wizard you will want to go into the Mapped Key it created and remove the Action where it turns off broadcasting. (To do so, select the Key Map in the top left pane, then expand the Mapped Key in the bottom left pane and select Step 2, then right click the Action in the right pane that says it's going to turn off broadcasting and select Delete)