New to real multiboxing

So I guess that I've "multiboxed" before...I have ISBoxer for controlling my screens when I play EVE, but I treat each window as if it were a separate computer with its own controls.

Anyway, I decided that I wanted to try multiboxing in WoW and I'm kind of surprised at how it doesnt work as I expected.
First, my set up - I'm a simple man, really:
1 computer, 2 clients, three screens.
Both clients are running the same class (i.e. 2 priests, or 2 mages, etc).
Main is on middle screen, secondary client on left screen, right screen for music, etc.
I pretty much just wanted to use ISBoxer to control my screens like I do in EVE, and have my mouse and keys cloned between them. My expectation was to have 2 toons standing in the same location, performing exactly the same actions at the same time (ms lag acceptable, but not expected since its the same computer). Thats it, nothing spectacular, I didnt expect to need any sort of macros or special keys or anything.

But this broadcasting thing is horrible! Within only a few actions, my characters are running in completely different directions. I reset them to get them facing/moving the same way...and we're back to complete disarray.

So now we get to my actual question: WHAT GIVES?!?! Why is it that cloning my mouse and keyboard doesnt really work? Its the same computer, and I can see that the two clients are receiving the same information, the mouse appears to be in the same location from client to client...so what is going on that makes it so that I cant actually clone my mouse/keyboard?

your broadcasting needs to be off that will happen if its on and keymapping is the thing you want on and the only reason to have broadcasting on is if like you wanted to turn in a quest at the same time

in wow you need to use a /follow macro to get your characters to stick together. trying to broadcast wasd is always going to be wonky because of latency and other factors.

As has been stated above, broadcasting all is not 100% accurate, so the characters will desync quite fast. As such, broadcast is only used for movement in very specific situations. More commonly you would use it to click on portals, menus, quest dialogs, walk through teleports that would otherwise just break follow, etc.

The way multiboxing in WoW works (for the most part) is that you lead from one character, and the others /follow it. Similarly you acquire targets for attacks by /assisting your lead character. All of this functionality is however setup for you "out of the box" by ISBoxer, so you don't need to be setting up macros just yet. ISBoxer will automatically handle creating an FTL setup for you, so that the same keys for Follow and Assist work from any character (up to the max of... 24 characters I think?).

Mirai covered this among quite a lot of other things in his Multiboxing Gameplay video series, which I highly recommend for anyone just starting out with multiboxing MMOs.