It's not very difficult once you get it down. I've gotten three dash 2s into 4,4 into nut punch lots of times in online matches.
I find the trick is to treat f f 2 as if it was a special move input. Don't look for the dash, then look for position, then poke, then look for when you can dash again. Learn the rhythm until you can literally do it with your eyes closed (literally), then once you're comfortable with that, use your eyes to tweak the juggle timing and choices based on the situation. For example you might be slightly late and want to replace 4, 4 with f+3 into nut punch so that the punch doesn't whiff.
Also I think Playstation pads are the worst controllers on any popular system. I'd rather do multiple dashes on an NES d-pad. I used to be terrible at quick dashes on a stick but they've been no problem at all since I got my Hitbox:
http://www.hitboxarcade.com/