I would steer clear unless there was some sort of condo declaration or CC&Rs that governed each unit owner's obligations and prohibits certain actions. I doubt there is any such documentation in place from the description you have provided.
There are so many issues that can arise with owning only 1 unit of a duplex with no such governing documents. I think some of the posts have already touched on the big items. There are access issues, insurance issues, damage/casualty events, activities you want to prohibit or control and joint capital improvement costs. The list can go on. You would really need some sort of agreement outlining all of this and recorded against the property. You would also want capital reserves from each owner in the event of such joint expense, such as a roof, driveway, or sewer line needs repair. Your roof example is a real issue and concern. Also think about joint walls, driveways, sewer and other utilities that may be shared.
Also, I would be concerned on whether the proper means were taken to legally separate the two units of the duplex before they were sold. This may be state specific, but unless a plat was done showing two separate units of the duplex, you can't really just sell half of the duplex. If you ever purchase a condo, take a look at the description of what constitutes the "unit." It is pretty much anything in the interior walls. Anything outside traditionally belongs to the association as a common element or limited common element. If there is any type of a yard or driveway, who owns that? Or are they separated as to each unit? Does each unit have a separate tax parcel identification number?
If there was proper legal documents in place, it could be very similar to owning a condo or townhome, but sounds as though this is not the case. I have seen several condos/HOAs with less than 4 units, so it is possible someone went through the hassle of doing it right, but highly doubt so in your instance.
