Playing Atlantica Online for a week or so now.

I made good use of HKN starting more clients, window swap them (PiP), removing frames of windows ...
You can party up to 3 teams (and with teams I mean max of 9 individual toons to a max party size of 27 controllable toons) and you can issue commands to 5 toons per combat turn (turns are time limited to 20 sec as well)

There is another thing called Squad (which I haven't tried yet)

BUT:
3rd party tools are forbidden and i doubt they encourage multi boxing.

(lower level bosses are 1on1 so no party possible there).

I currently used it for having on main in a city and mailing items back and forth, since bagspace is severely limited. Mail is instant though.