Here's the thing that most people never realize and dualboxers live with constantly...

You target something and hit attack, your alts dont know about that target for about .5 to 1 full second.

So if you have a single button to assist and target, they wont know about it, I have gotten in the habit of spamming the button and I have party-castbars on screen so I know when the rest of my groups casting... ToT indicators let me know that they are on the right target, etc.