Great point. These two were not eyewitnesses but earwitnesses who disagree with the eyewitnesses.
Yep. DeeDee is necessary to introduce evidence that Zimmerman was in hot pursuit of Martin, essentially that Zimmerman was the one who chose to close the distance, and was able to do so. DeeDee is also the only source for the contention that Zimmerman initiated physical action. There are many problems with her as a witness, and it's obvious she is either speculating or lying as to who shoved who, if she heard anything at all at that moment.
Sybrina's testimony is used (and Tracy Martin will do the same, although he's gone both ways on the point) to establish that the persistent screams for help come from Martin. Her testimony is directly contradicted at least by eyewitness John, and may be contradicted by the difference in voice quality between Zimmerman and Martin. The way I see it, take Sybrina and Tracy on one side, and Zimmerman's friends and relatives on the other, and they balance each other out. So, eyewitness John, plus the circumstances of Zimmerman being on the bottom, Zimmerman saying he yelled for help and nobody came to his aid, a one minute+ fight, 14 screams for help ... pretty easy to conclude that Zimmerman is the one yelling, beyond a reasonable doubt.
The state's case is pathetic, and as far as I am concerned, was brought unethically.