There are several methods you can use.

I like to have a mapped key.
No hotkey.
Target is Current Window.
And it does whatever:
a) Keystroke Action (macro, in Warcraft); or
b) Named Warcraft Macro Action (macro, stored in IS Boxer)


As an example...
Mapped Key: Melee IWT
Action: Keystroke Action > My WoW IWT Keybind
Target: Current Window




Then you have a mapped key you will activate via a hotkey.
Or another mapped key.

Decide which toons want to use a given mapped key.
Decide by Slot number.
Or by Character Name.
Or by putting toons into logical groups, such as all your Melee into an Action Target Group (ATG) named Melee.

So this mapped key calls the previously made key.
Do Mapped Key Action > Melee IWT > Target: Group (all of) Melee [The ATG with Melee in it].

The mapped key that is calling the others, determines the target or targets.
The called mapped key, is current window only.
The calling mapped key, calls the mapped key for every target you would like it to execute for.

Hope that makes it clearer.