Why wouldn't he be allowed to do both? Star Trek 3 if there is another won't be till 2015 or 2016 plenty of time To do first Star Wars then the last trek then back to concentrate on Star Wars.
It just seems like there would be a conflict of interest. Perhaps I'm wrong. Logistically of course he could direct both. I was more referring to there being a creative conflict. While not identical, both franchises have a lot of similarities. I found Abrams first Star Trek movie to be reminiscent in parts of Star Wars.