If you plan on really boxing, then you need to incorporate Shift 0-=, Alt0-= & maybe even Ctrl0-=. Just using 1-9 isn't going to cut it. I 6-box two different teams. I've got my keys down to a pattern across all my toons:
1-6 : DPS/Atks
7 : Everyone stop and follow tank
8 : AOE_Encounter atk
9 : AOE Atk
0 - =: Special functions
Shift 1-6: Heal toons 1-6
Shift 7-=: AOE Heals and Superheals for oh'krap moments
Ctrl 0-=: All the extra special things each toon can do, like mezzing or charming or whatnot

I then remapped my logitech mouse buttons to match up with the Shift1-6, so I can effectively kill with my left hand on the kb and without skipping a beat, heal with my right hand on the mouse. I normally run 2 healers in my group (unless the pally is tanking). For all the non healers, shift 1-6 either protect the healers, or activates de-taunts for dps chars, and the like.

It has taken me somewhere around 100 hours of tweaking and trying and flexing and messing up and fixing and such, to come up with my system. I have also created custom /load_keymaps, created for DPS or Healer or Tank toons.

To my knowledge, there is no way to send a particular key to only 1 window... the closest you can get is Round Robin, but that won't do it either. I recommend the shift1-6 method, and then have shift1-6 on all your non-healer toons just be the HO Starter.

I've taken one 6-man team to lvl 78-80 and my 2nd team is already in their low 30's, only because i'm disabling XP so I can hit every instance and dungeon in the game with my 2nd team and maximize AA gain. It's not easy to setup... but once you get it going, it's a blast!