Assholery aside, I know plenty of people who only listen to one genre. And even if it were a bunch of metalheads who listen to multiple genres, most of the people would be listening to different, uh, 'secondary' genres - meaning that possibly only 5% or 10% of the audience would like both Post Rock and Metal.
I'd love to see both of these bands, but, in general, they appeal to pretty different audiences.