If you have two compressors (the outside part) and two air handlers (the inside part) they are completely separate systems and can be replaced together or one at a time. Mini-split compressors can support three inside units.
Some air to air exchangers are stand-alone units and some are built into the duct work of the HVAC system. As houses get tighter they become more of a necessity to bring in fresh air without using a ton of energy to condition the outside air to the desired temperature. If your house is exceptionally large (3500+) it might need two, but I'd bet you could get by with one for a smaller house. That said, I'm not an HVAC professional. I could ask an old classmate of mine from HS who owns a HVAC business in MI for his opinion.
You mentioned nothing about wanting to put an air to air exchanger in your original post. Don't make us tease out the true question through multiple posts.