Your comment matched my understanding from ~10 years ago (from discussions on CA motorcycling forums). However, AB-51 passed last August and it's added "lane splitting" to the vehicle code (which I just found out about 10 minutes ago).
Anecdotally, I'd heard of motorcyclists cited for violating the CA Vehicle Code if they occupy two lanes: "Existing law requires ... that a vehicle be driven as nearly as practical entirely within a single lane and not be moved from the lane until the movement can be made with reasonable safety."
This resulted in the semantic distinction. It's not illegal for two vehicles to share a lane. It is illegal (and still is) to drive in two lanes or to drive "between" the lanes.
https://leginfo.legislature.ca.gov/faces/billTextClient.xhtm...
Anecdotally, I'd heard of motorcyclists cited for violating the CA Vehicle Code if they occupy two lanes: "Existing law requires ... that a vehicle be driven as nearly as practical entirely within a single lane and not be moved from the lane until the movement can be made with reasonable safety."
This resulted in the semantic distinction. It's not illegal for two vehicles to share a lane. It is illegal (and still is) to drive in two lanes or to drive "between" the lanes.