Again I'm not an HVAC tech, but my research shows yes they can be installed separately. Here's another reference: https://yourairexperts.com/blog/furnace-and-ac-connected/#:~:text=Not%20all%20homes%20will%20have,no%20furnace%20or%20vice%20versa.
"Understanding the difference between a forced air system and a central air system can help you differentiate the roles of each in your HVAC system. Essentially, a forced air system encompasses the entirety of HVAC systems that move temperature-controlled air into your home with ducts and vents. Examples of forced air systems are your furnace or your heat pump (if you have one).
Central air conditioning uses a closed loop of refrigerated/cycled air to deliver cool air when needed to combat the North Carolina summer heat. Both your forced air (furnace) and central air (AC) systems use the same parts to cool and heat your home.
The outdoor condenser and compressor, indoor fans, and evaporator coils cycle refrigerant and draw hot air inside while releasing heat, then draw cooled air back through your vents.
In other words, your central air conditioning system is independent of your furnace. The outdoor unit isn’t connected to the furnace at all — but they both use the same distribution system (vents, plenums, and ducts) to cycle cool air into your home."
I think that last part I underlined is the key that makes me feel the furnace and central air system are 2 different units that each qualify for $600. You could install or replace your AC one year and your furnace a year later. Although typically people replace the whole thing at the same time, they are two different units, although they use the same distribution system.
I don't know how things are in other parts of the country but here in IL, a furnace inside the home and an a/c outside in the backyard are a split system. And one can be installed completely separately from the other. If they each otherwise qualify, you have up to $1200 in credits.
Think of it this way. You could do the ac in 2023 and because the cost is obnoxious, hold off on doing the furnace until it dies in 2025. No question there about two credits.
As for the paperwork, see if they can write it up as two projects. Or if not, allocate the labor on some reasonable basis. Or maybe the cost of the units themselves are such that the labor component is irrelevant.
