You could split the cost and then get a credit for the A/C unit and a credit for the furnace. However, since the cost of the HVAC was not broken down by unit, if the IRS ever questions either credit you will not have proof of the cost for each.
I have the same issue as I replaced by HVAC in 2023 and the unit is eligible for the credit. But I only entered the total cost as an A/C unit since I did not have any paperwork to show separate costs. That was my decision.
