Legal Question in Real Estate Law in California

In most cases, yes. Separate APNs almost always indicate separate legal parcels, and separate legal parcels can almost invariably be sold separately. On rare occasions separate APNs are attached to parcels that have been subdivided in violation of the Subdivision Map Act, or for other reasons are not legally separate parcels. So separate APNs are a good indicator that there are legally separate parcels that can be sold separately, but it is not a guaranty.

95% of the time, parcels with different APNs are "separate" for purposes of buying and selling. However, occasionally the parcels cannot be sold separately because either (a) they have been merged in deference to some county policy (zoning, etc.) or (b) were never legally split in the first place. Some counties have been more aggressive than others in legally merging adjacent parcels in common ownership. Occasionally, these mergers are not completely legal and the owner(s) can file suit to have the supposed merger nullified. See Miller & Starr's treatise on California Real Estate, section 25.14, "Merger of Contiguous Parcels" and Civil Code section 1093 at your county law library.