The first double carpool lanes in Northern California are going to be built in a 3 mile stretch on 101 between 85 and Embarcadero in Palo Alto. These are extremely rare (had a real hard time finding a photo) and exist on just a few freeways in SoCal.
Eventually these will become double toll lanes, which is part of a much larger project to create double toll lanes stretching from Morgan Hill through San Jose and up to San Mateo. Toll lanes allow single drivers in the carpool lane for a fee that increases with higher congestion. So far the only toll lane in San Jose proper is along 237.
Highway 85 is also going to get the double toll lane treatment as soon as 2015 and 880 is going get it's first carpool lane in each direction between 101 and 237.
Source: Marin Independent Journal