Here are some screenshots to make this hopefully clearer. I'm using just 2 of my normal 5 chars:
- Pally Iaggo = F1
- Shaman Katator = F2

I actually don't even have them grouped. It doesn't matter under this system, though of course you would want them grouped once you were questing.

Hotkey setup:


Default setup with Iaggo as main:


Switch to Katator as main: