Yes, coordination is complicated. The need for, and difficulty of, coordination is why I believe there was just one shooter.
1. The shooter does not have a history of collaborating with others in matters of importance. For example, he gambled alone.
2. When you listen to the 10 minute period when he was shooting, there is more down time than this is shooting time. If there was more than one shooter, one would expect for there to be more actual shooting time.
I believe the shooter was narcissistic, and thought he was better than anyone else...a trait which ultimately contributed to his doing the deed. No way he would have let anyone else in on his plan - the preparation and execution was part of the allure.