Dual rooftop Systems for residences in the Southwest

I recently took a vacation to Arizona, and I noticed many of the single family homes have a small packaged conventional Cooling Unit (with electric heat in it I assume) coupled with a Swamp Cooler tied into the same supply duct as is visible from the outside .
My question is : Is there some type of damper arrangement at each unit that will prevent the supply air of one unit that is operating from escaping thru the other unit which is dormant ? And, is this controlled thru automatic controls within the dwelling ? Maybe an HVAC guy from the Southwest can enlighten us on this setup. Thanks.
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