That's my guess. Two shooters. Makes it less likely for one to turn the other in if both have killed? One may be better than the other, thus does most of the shooting.
I am virtually sure it has been voiced elsewhere, but if the witness accounts are certain, then the more cars, the more likely that more than one person is involved (imho). If more than one person, the getaway driver has probably been waiting patiently or otherwise for his or her turn.
Someone local might be able to check with any local policeperson for how many vehicles they are looking for, and opinions on the implications if any from the apparent variation in demonstrated skill level, since all my thoughts are just from the reports on FR and the news only...