I have a lot of problems with that exception. First of all the "dual sovereignty" exception is completely an invention of the courts which came about as a result of a 1959 decision by the United States Supreme Court. There is nothing in the Fifth Amendment, which contains the double jeopardy prohibition, that suggests there are any exceptions that would allow dual prosecutions.
When the Fifth Amendment was adopted there was little in the way of federal criminal law. Development of a criminal code was the province of states. The federalization of crime is a fairly recent phenomenon which has been increasing at a tremendous pace the last few decades. Now for every state crime you can often find a comparable federal crime for which the person can be prosecuted. Thus the opportunities for dual prosecution have exploded.
The United States Supreme Court has not had a "dual sovereignty" case for nearly 30 years. The game has changed completely since that time. I suspect there would be a strong left-right coalition on the Court who would vote to overturn the doctrine which is an anathema to criminal defense attorney, constitutionalists and civil libertarians.
But even if a second prosecution is possible, it doesn't make it right. In prosecuting individuals, the government is supremely powerful. People accused of a crime often have their lives upturned for years trying to defend themselves. Often the prosecution tears apart families, the defendant can't get employment, and many times the defendant ends up bankrupt paying his defense attorneys. When a defendant is acquitted, all that money he paid for attorneys to secure that acquittal is gone. Even in the case of acquittal, the person accused of a crime often has to spend the rest of his life with a tarnished reputation simply because he was charged with a crime.
In light of the tremendous power of the government to destroy people's lives by bringing criminal charges, it is extraordinarily unfair to give government a second bite at the apple. The fact that it is a different government means little to the criminal defendant who has to defend himself. The focus of the double jeopardy protection should be on the individual defendant, not on the government infringing upon that protection.
Whether one believes "justice" should be brought to bear on George Zimmerman in a subsequent federal prosecution, hopefully people, especially my fellow attorneys, can see the danger in such a prosecution If a federal prosecution is pursued against Zimmerman, there is no reason that federal prosecutors can't do it in other cases where state court defendants were acquitted. A second Zimmerman prosecution, following an acquittal, would do great harm in undermining our system of justice. The greatest impact of such dual prosecutions will be on the poor and minority who are often the target of an unfair criminal justice system. By arguing for a second prosecution, groups like the NAACP will end up hurting the very people they claim they want to protect.