When you press more than one button at a time in CvS2, the game will streamline your input into a single button press in most instances. For example, pressing jab+strong+fierce will result in just a fierce. Now, for the longest time I had been under the impression that higher strength buttons had priority over lower strength buttons, and after that kicks had priority over punches. Some more examples: pressing short+strong will result in strong, pressing fierce+roundhouse will result in roundhouse, and pressing all 6 buttons will result in roundhouse also.
I was under the impression that these same rules applied to special moves also, like a hcf+jab+short would always result in a hcf+short. Well, I was wrong.
When it comes to special moves and supers, CvS2 seems to go by some sort of arbitrary rule on whether a punch or a kick comes out. According to my previous logic, a qcf+jab+short would always result in a qcf+short. You actually get a qcf+jab. Now comes the weird part: do a qcb+jab+short, and you get a qcb+short. What the hell?
Anyway, after some quick testing, here’s a list of moves and whether you’ll get a punch or kick move if you overlap the buttons:
rdp: cannot be tested, since no character has an rdp move with both punch and kick
charge b,f: punch
charge d,u: punch
hcb: punch (note that this is different from qcb)
qcbx2: punch (?!)
charge b, f,b,f: punch
So what does all this mean to you, the aspiring player? Most people agree that the easiest way to roll cancel jab/short moves is to double tap roll while doing the motion. This little chart here tells you what you can and cannot roll cancel by doing the double tap method. For example, if you want to do RC jab Blanka balls, that’s doable. RC short scissor kicks? Good luck (unless you’re Japanese and know some kinda trick I don’t). You cannot RC Gief’s jab SPD for extra range (again, not using the double tap method). Rock’s jab elbow? Nope. And you can figure out the rest yourself. Of course, you can sometimes get the “other” special move to be RCed with the double tap method, but I find it’s more of a random occurence than not.
Also, remember that you RC any move with the double tap method as long as the character doesn’t have another conflicting input in the way. For example, Ken’s short funky kick can be RCed using the double tap method since he doesn’t have a hcf+p move to get in the way.
The chart for supers is mostly useless, but I just added/tested it for the sake of completeness.