I'd contact my local plumbing inspector to find out what's required in San Diego.
In my building I have three toilets (one on each floor) in an apartment building all discharging into the same stack. For example, the toilet in suite 3, suite 10 and suite 18 all discharge into the same vertical cast iron pipe.
However, if your house was built with each toilet discharging into it's own stack, then there's a difference in what the plumbing code requires where you live. And you have to go by your local rules.