Normally a duplex would be considered two separate dwelling units. IRC 168(e)(2) doesn't contain a very specific definition, but does include the word "apartment" from which I conclude that a single building can contain more than one dwelling unit.

(I) the term “dwelling unit” means a house or apartment used to provide living accommodations in a building or structure, but does not include a unit in a hotel, motel, or other establishment more than one-half of the units in which are used on a transient basis, and

https://www.law.cornell.edu/uscode/text/26/168#e_2

The term dwelling unit is also referenced in IRC 280A and the regs provide a much more comprehensive definition, although not specifically referenced by IRC 168.

Therefore, I would treat your situation as one dwelling unit would be a long term residential rental with a 27.5 year class life and the other would be commercial or business property with 39 year class life.