Re: Huh. Well this is interesting. (Metal content.)
When the main riff kicks in again around 1:00 it does seem obviously doubled. In the conventional way. It does sound to me like the bass is doubled throughout- and it's tight enough not to eff up the groove, but it does sound like two takes of bass (aside from any possible overdubs like the first chord slide toward the end), which is interesting, in that it's not something you ever really hear about anyone doing.
Chris may be right re: it being the way it's played as far as the upper octave thing goes. It's enough to make me wonder at first listen (the vibrato makes me assume it's not an 8-string bass) though. There's some cool shit going on here sonically though.
As an aside, re: how it fits together, I actually unearthed this on the Electrical Audio forums with the comment "Pantera - Dimebag = Shellac".
I can hear that. The chug sections and to an extent the bass tone as well, though I think of Mr. Weston being a bit dirtier tone-wise and way more chord-y in his playing.