Usually that means that there are two houses on one lot. In otherwords, it is a duplex but instead of being attached units, they are separate which is a huge plus. You say that is the only one you have seen like that in your area. In my area, that is quite common, though not every lot is zoned for multiple units. Since it appears to be rare in your area, you might want to check and see if that lot is zoned for multiple units. Do your due diligence if it is a property you are interested in so as not to have any surprises down the line.