Im really new to dual boxing but I just made a macro like this on my secondary character

/assist "main characters name"

I just Tab my target on my main, then press this macro and he targets the same thing as me, I do notice though I cant do it real fast I have to wait about half a second after I target other wise It dosent pick it up, im learning to play as I go. Its funky at first but I found after a few hours it becomes second nature and ya dont even think ya just do it auto.