What your looking at is a fine set up.
Duct furnace on bottom with a 5 ton coil backing it up. Then on top of it there is a super flex connected XTRA air handler with 3 or 4 more tons of XTRA air conditioner for EXTRA cooling in summer. All pumping into 3 ton ductwork. They told me it worked fine for 20 years when I said it might have install related issues. So I condemned it for it's cracked heat exchanger. The fan limit is not a good thermostat.
New definition of works fine- workers go to attic in winter to warm up.(the furnace radiates heat so bad since it lives on the edge the attic is hot!)
Works fine also means you slide plywood into filter door on the EXTRA unit and shut off breaker when not in use to prevent air from short circuiting.