First problem.

There's been many suggestions of a second shooter on a different floor. This would seem to me to be impossible because the windows can't open. If there had been a shooter there the windows would be blown out like the upper floors.

Second problem

There's been audio that sounds very much like a second gun firing. However the reports are that he had like 20 different guns in the room. A .22 cal rifle heard right after a .223 could sound like the same gun further away when actually it's a different gun in the same spot.

Also (correct me if I'm wrong) the audio doesn't sound like 2 guns at the same time. It can be difficult to tell because of the crack of supersonic projectiles plus the muzzle blast are both from the same gun.

Just some thoughts, Happy to be refuted