Your link doesn't support your claim, as it indicates rent control exists in D.C., several cities in Maryland, over 100 jurisdictions in N.J., as well as a smaller (but still large) number of jurisdictions in NY and almost a dozen in CA (there's also some more in CA listed in a heading of “other than rent control”, which seems likely to be intended to indicate something that is not rent control but raises some similar issues for landlords, but the links all appear to be dead links, so the actual specific meaning is less than clear.)

So, if it's “almost exclusive” to any pair of states, it's New Jersey and New York, not NY and CA.

(The source is also inaccurate in at least one respect, in that NY by the description given on the page and CA due to Costa-Hawkins both meet the criteria for a “Yes” in State Prohibits or Preempts Rent Control column, but both have blanks in that column.)