@Christopher Davis - @Steve Frye is 100% correct in all his advice. "Two family dwelling unit" is not exclusive to an umbilical HPR. What Steve Frye explained is correct and true, separate entities, but 1 permit because they are on the same lot. Ownership is not divided into 2 distinct lots. It is still one lot.
Think of it this way. HPR is kind of like an HOA w/o the fees. Same property as always, but it has been divided into 2 separate units owned that can be owned by the same individual or separate individuals. They share "common elements" and Limited common elements" similar to an HOA. So the grass, driveway, fences, yard etc. are technically shared. Most builders will separate them making it easy to identify which homeowner takes care and maintains which portion of their pie.
An example would be an HPR-attached that shares the same roofline. That roof will need to be replaced one day. It will be hard to replace seamless roof without doing the whole thing. Most HPRs operate as such depending on their language where each homeowner has to agree to fix, change or repair to those "common elements". It will interesting to see 10-15 years from now when roofs need to be replaced how homeowners will handle that. Who will pay for it?
I would go to parcel viewer click on any HPR and look at the HPR docs and their specific language. Everyone is different, but they essentially operate that way. It's not a perfect example, but it at least gets your head wrapped around something most people are familiar with.