Re: Need assistance dunking off of 2 legs.
A couple of routine two-foot takeoff suggestions:
1) Keep your dribble low (and subsequently, keep your body low): It might be a good idea to take some dry runs to the rim (i.e. without a basketball, as you mentioned you do). Only, concentrate on when and where your body needs to be in order to maximize your takeoff. It might even help to film yourself jumping to the rim without the ball then filming yourself trying to dunk with a ball.
Oftentimes, the basketball can really mess things up due to a high dribble or other upright tendency that tends to zap a person of their normal springiness.
2) Stride it out, put your dribble in front of you: The idea is to duplicate your non-ball jumping routine for your actual dunk attempt. Sometimes I find slightly leading the ball out ahead of me (while not dribbling too hardly) can allow me to prepare for my normal, low, explosive takeoff. Getting low to the ground prior to takeoff is essential.
3) Start with a smaller ball: This is not something I've ever personally done, but it seems like it could make sense. Just in terms of maximizing your mechanics, it might not be a bad idea to give a smaller basketball a shot, just to get a feel for what success will look like.
4) Stay on your toes: Or more to the point, avoid setting strongly on the heels, thus zapping your leaping momentum.
5) If one-handers is all you've tried thus far, attempt the two-hand flush.
Above all else, as lame as this may sound, it may just be a matter of needing to add a few more inches to the ol' vertical (easier said than done).
My last thing I'll say though, if you don't already, try maximizing your running workouts. Sometimes I catch myself trying to preserve energy during pick-up games in hopes of saving up for a dunk opportunity at some point. However, it seems doing the opposite (going all out) is what provides me with the best opportunity. The benefits are two-fold: 1) Sprinting increases stride and stamina and 2) Running very hard then taking a short break seems to activate the leaping muscles. A lot of times, we can jump higher when we think we should be tired instead, if that makes any sense.