So, treat primary residences as primary residences, whether or not they are the primary residence of the property owner. For multi-unit rental properties, the total value of the property is divided among units proportionately to the rent charged for each unit to assess this.
While it's probably not a bad idea, I don't see it moving the needle much.
I'm discussing ways to achieve the goal upthread of insulating people from property tax uncertainty on their primary residence.
I would expect that the larger purpose of that is to allow full-value taxation on all other real property; of which non-primary-residence residential property is a subset (and a fairly small subset, at that.)
Remember that Prop 13 applies to all real property, not just residential property.