Ductwork in a duplex

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Hi everyone, I’m wondering if someone can shed some light on whether air from one unit in a duplex gets circulated through the other unit or not. This 2 family house has central air and gas heating. There is an outdoor condenser unit for each apartment and one single furnace in the basement. The reason I’m asking is because one tenant has a cat. I’m wondering if the dander would transfer into the other unit via ducts and cause potential allergy problems for that tenant. Thanks