There's several things to look at here and it *does* matter big time.
- Is each unit separately metered separately for utilities? (Gas, Electric, Water, Sewer, Cable, Internet, etc.)
- Is each unit a "ground floor" unit, or is one unit upstairs? This matters for figuring land portion allocations in some (not all) cases if it's an upstairs/downstairs setup.
- Are the two units identical? (If unit B is a mirror image of unit A, then they are identical). This matters for determining the percentage of floor space that is allocated as "exclusive" to the rental.
- If both units are ground level, is the structure located in the center of the property between the two property lines? This can matter for determining the amount (and value) of the land that gets allocated to each unit. It can also matter if you elect to sell one of the units while keeping the other.
Now I'm sure that you would receive one single property tax bill for both units, as well as have only one mortgage and only one insurance policy for the entire structure with 2 units in it. How all that gets split may not always work out 50/50.
Additionally *AND THIS IS IMPORTANT!* a standard homeowner's insurance policy *DOES* *NOT* cover rental property. So make absolutely certain that if you intend to rent out one unit that you ensure your insurance agent knows this and confirm that the rental property is covered.
Generally, a homeowner's policy will only cover the owner's primary residence and/or 2nd home. It does "NOT" cover business property - and any way you look at it, rental property *IS* business property.
For rental property you need what is referred to as a "Rental Dwelling Policy". So if in a duplex and one side will be personal use property, I don't know how things will work on the insurance front. So you will have to work this out with your insurance agent.
Basically, an insurance company can legally (and quite easily) get out of paying on a policy if the incident that led to the filing of an insurance claim occurred while the property was not being used for it's "insured use". So I can't stress the importance of having the right type of insurance on this.
Overall for your situation, I would suggest that on your taxes you file and treat this "as if" each duplex was a physically separate piece of property. Your life will be "much" easier when it comes time to sell and report the sale on your taxes. It also simplifies the "capital gains tax exclusion" process if you live in one unit for at least 2 of the last 5 years you own it.
