Why does XFIINITY have 2 different PAC-12 channels and both are SD and have no HD version?

There are seven different Pac-12 channels. Each area with a Pac-12 school is "supposed to" get the main one and its regional one (Washington, Oregon, Bay Area, Los Angeles, Arizona, or Mountain (Utah/Colorado)). Most of the time, they show the same thing, but occasionally they show different sports - for example, as I type this, the main channel is showing Stanford-Colorado volleyball, while the Bay Area channel is showing California-Utah volleyball.

As for why you don't get an HD version of the national one (I'm not sure HD versions of the regional ones even exist), your guess is as good as mine; it's channel 823 where I live.

In the Seattle area Pac-12 Washington is available in HD on channel 1330. It is an IP channel. The national Pac-12 Network is in HD on channels 628 and 1329.