Hmmm. I guess you must be talking about non-integrated heat & A/C systems piped into the same ducting. Yeah, I guess there could be some loss from system back pressure of heat into A/C units and A/C into furnaces, etc.. Perhaps just a standard 45 degree in-line junction damper where the heat enters the ducting. By simply switching the lever, it would open one while simultaneously closing the other, or, install a backdraft damper in the line of each of the A/C and heat sources like pictured. If you do rectangular ducting, use a vent hood backdraft unit.
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Larry
"When life gets hard, eat marshmallows”.
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