Possible Bump Fire.
First one was a 50-75 rd drum while music playing. Very slow reload to the next magazine. Probably not a second shooter which would start firing when the other shooter stops. Very VERY slow reload. Probably a slide fire stock or auto-glove that makes semi's bump fire (similar to full auto) and hard to reload.
There is a video on twitter that makes me believe this is an auto glove due to variable ROF (rate of fire) going up and down. Either that or there were two rifles being shot at once.