Take a look at the "Others Jump", mapped key (in the General Keymap).
It has the Other Windows do something, instead of the main window.

You can copy that, but change the target.
I'd go with WASD and the Arrow Keys (Up, Down, Left, Right).
One set is not mapped at all, which means they act as normal for the current window.
The other set is mapped, similar to "Others Jump", meaning the other windows receive the keys but not the currently active one.
So no matter what window you're in, the other windows can be manually moved.

Also consider Left/Right arrow, sending Strafe Left and Strafe Right (if you primarily PvE).
If you primarily PvP, then turning the slaves might still be preferable.



You can make a spread out key.
Give each of the windows a different output, all on the same hotkey.
One toon can get W + Q.
Another can get W + E.
And then give two of them, S + Q.
And S + E.

That's forward and left strafe, forward and right strafe, backwards and left strafe, and finally backwards and right strafe.
You might need to select the option to Hold Keystrokes while Hot Key is held.
So they move, in the direction configured for as long as the hotkey is pressed.



I have another for a horizontal line.
Two toons receive Q, and two receive E, on the first Step.
And on the second step, one of the two Q's receives Q but the other does not.
And ditto on step 2 for the E toon's, with one receiving and one not.

I press the key a little.
And release.
Then press the key about the same duration.
And release.



You can do a lot of combinations.
Each step is a different output on subsequent presses of the same key.