I've never seen this in other bands, in the ride the lightning album was the same thing, in certain point, he continues the main riff but then starts to play other different riff at the same time, that's impressive. The same thing happens in the for whom the bell tolls intro.
As a teenager, I swore this was a guitar....but no. It was the Major Rager on the Four-String Motherfucker. Cliff did insane things with his bass, the band would have been completely different had he survived.