Fwiw, just holding the ball at two seams isn't going to do anything.
I'm going to go ahead and disagree here. If all else is the same except the orientation of 4-seam vs 2-seam, the 2-seam will have much less "bite" on the air, by which I mean that a 2-seam will have slower flowing air under the ball than a 4-seam due to fewer seams in the air flow around the ball. This means a 2-seam will have greater air pressure underneath than a 4-seam, and so less drop (assuming we're talking about a 12-6 rotation). I'm not sure that having a pitch with less movement is desirable, but that's what ought to happen. To be honest, I'm not sure that the short time of flight in fast pitch would allow for a noticeable difference?
Now, if I understand the whole finger pressure thing (and I probably don't, so be gentle), the finger pressure helps to determine the yaw orientation of the ball as it travels? If so, it would make sense for finger pressure to be the key to getting the ball to move east-west.