When installing more than one air conditioning unit it is important to keep in mind the distance between the units in mind. If the units are not distanced properly it can cause shorted life span and cause problems with the air conditioning compressor and other components. The recirculation of heated air being absorbed by the condenser can cause the unit to over heat unnecessarily. It is important to keep the area around the unit clear of blockages, trees or debris.
Remember that in order for your HVAC system to perform at its peak it has to have fresh air circulating through. A properly installed air conditioner will last the life span that it is designed to last, but proper distance between the units is a need in order to even have proper operation.