Setting two buttons to specials probably wouldn't help, methinks. You'd end up overlapping button press and button release events from both buttons you set to special. I don't believe brawl is set up to 'count' inputs in those cases...
Also, I'm with Elh on the vibrating fore-arm thing... Except, whenever I played a game requiring uber amounts of button pressing, I whipped out a pen and slid it rapidly onto and off of the very edge of the button I was assaulting. By using the pen, I got rid of the friction between my fingers and the controller. By extending the radius from my elbow, I didn't have to vibrate as far. The result of doing both: less distance equals less momentum equals less power necessary to change direction equals higher frequency. :-D Well, that's the case as long as the pen doesn't get caught on the edge of the button... :-(
Too bad for me, though; it's kind of hard to really quickly bring out a pen in the middle of a game. <_<