Based on the pictures I've seen, the one window angle may have been somewhat obstructed by the large concert screen. The corner window didn't seem to be as much. Also, the crowd would most likely move directly away from the stage, into his second field of fire.
Alright, I'd say, except two nests. If he got all set up, and was running out of targets, and moved to the secondary, it would have been an ad-hock/impromptu/make do station. He purposefully built two nests.
He's got some hunting experience, so that explains the high perch, but predicting a requirement for two seems odd.