First off, if it was between gang members, each would have a sign of membership, usually a tattoo, in a visible site. If it were premeditated and gang-related, the most obvious evidence would be for the cuts to be on the marks.
Also, the attack on the man in custody (hereby named Bob) could easily have been out of anger, due to the placement of the cuts on his palm. Chances are that he raised his hand in self defense and missed the attacker's wrist. Had it been premeditated, though, the attacker would have wanted to strike Bob in a more vital place, not his hands. He would have gone for the back or chest.
Continuing, if there was only one knife, I wonder how long it could have taken from each to do this to the other. My theory is that there was a third perpetrator involved who premeditated the act and dropped the knife on his way to flee. Check the knife for fingerprints and cross-check them with the victims. Also, make sure to check the blood on the blade, for the victims may have struggled and struck the third guy in defense.
