The divisional alignment for the soon-to-be Pacific 12 has just been announced. As seen in the following screen capture from the league's press conference video, the split is very straightforward.
The South Division will consist of the two Arizona schools, the two Los Angeles schools, and the two new schools (Utah and Colorado). The North Division will include the two Bay Area (California) schools, the two Oregon schools, and the two Washington schools. Although UCLA and USC will be in one division, and Stanford and Cal in the other, all of the football rivalries within the state of California will be maintained. As noted in the above-linked article:
"To preserve all of the historic rivalries, including those among the California schools, the new scheduling format locks in annual inter-divisional games between the Northern and Southern California teams, that date back to the 1930s."