Although I'm sure this is accomplish in other methods. I first taught myself how to dual-box manually playing 2 separate characters at the same time with no keybroadcasting, then moved up to 3 then 4 in my travels ( with key broadcasting now of course

If you look on the bottom left there is 2 nostromo n52 gamepads. The keyboard is just for the main pc and does not broadcast any keystrokes, my 2nd pc which is connected to the middle nostromo gamepad broadcasts to pc's 3 and 4, so when I am multiboxing I am Always pushing 2 buttons at the same time, when I need to turn independantly I turn my main and the followers any which way they need to go by this method.
Of course this method of control is not exactly efficient (losing mouse turning ability, others call me a keyboard turner...) it gets the job done for what I need to do. It also takes some time to master since your brain doesn't enjoy attempting to process 2 separate movements going opposite ways.