$18,000+ seems a little steep? Seems you could get a volume discount if your doing both. About 3 years ago I had to get a new furnace , I decided to ‘add’ AC as well. On top of that our city building ordinance changed and I could only vent one item into our chimney. Since I have a small fire place that I feed into the chimney, I needed to vent my hot water heater another way.. Ended up getting a high efficiency water heater as well that vents thru the wall outside. Anyway, my total cost for all was just under $8k , which included the new water heater and wiring for the new AC. Figured I am also saving money with the higher efficiency furnace ( winter months about $20-$40 month). Since you already have AC your wiring should be all set(No extra $)..
Since your units are older (1-over 20 years and 1-just under) ... you may still end up having to replace them sometime in the near future? If you could get a good volume deal on 2-units and get the higher efficiency furnace to bring heating costs down it may be something to consider.. Not sure if your renters pay for their own heat, but if their heating costs come down and they are great renters you want to keep, lower heating bills for them may help retain them ?