I guess that is where my confusion comes in, as I'm not an HVAC tech. Lol. My understanding is that a Split system refers to the fact that part of the system is outside, and part inside.The air handling unit is inside in a closet, but the condensor and other components of the AC are outside the house. The furnace is usually attached at the air handling unit inside as well. But concevably you could have an AC unit without a furnace and it woud still be a split system I believe. You could also probably replace the condensor and air handling system of the AC, and not replace the furnace. I could be wrong on all of that but this is why I have the question in the first place.

This is from an HVAC  website https://www.carrier.com/residential/en/us/products/air-conditioners/what-is-split-system/

"A split HVAC system is an air conditioning system or heating system that has both indoor and outdoor units that are connected with copper tubing.

Traditionally, the outdoor portion of the unit contains the compressor and condenser, and the indoor portion of the unit contains an evaporator coil and indoor air handling unit that sends the air through the ductwork in your home. These types of HVAC systems are different from HVAC packaged units, where all parts are packaged together in one unit.

There are many different types of split systems, which can include heat pump systems or air conditioning systems (larger homes might require multiple air conditioners) installed outside your home, paired with either a gas furnace or a fan coil located inside your home."

Further the IRS instructions say this

• $600 for central air conditioners.

• $600 for natural gas, propane, or oil water heaters.

So my thinking is that, although it is all one connected  HVAC system, it is made up of 2 different components: a central air conditioner and a natural gas furnace, so each of those components should be a separate $600 credit. But I've been unable to find anything to confirm or deny this understanding.