You want to hit the meatiest part surrounding the stud on opposing sides at exactly the same time, and really f***ing hard.
Sometimes you can't get straight onto opposing sides, and being a little off doesn't hurt. Sometimes you don't get it timed right, especially with different weight hammers, but that does matter. It's the magic of making both sides of that metal deform at the same time. If you do it right, the taper will pop loose and eject in that split second before the steel springs back to shape to grip it. You'll know if you hit it right because the hammers will kick back nicely from each other's impact.