It's not that they have multiple mouths, they have multiple sets of teeth, so one set can hold on and the other set can get a bigger/better bite of the pray!!!
I heard eels had a good bite. You could base an alien movie on these animals.
I know life is short, brutal and savage, but it is something I never get used to, especially the idea of being conscious while being bitten in half. When I miss Game of Thrones, I'll rewatch this video.