CTO 
3500 cfm is probably open discharge ( no duct work resistance) They can't be pulling all of the condenser air from inside of the building unless they are just spot cooling an operator .In you case I am thinking 1 (18") is intake from outside and the other is exhaust to outside.Check with your source on this. 
   Like Bob said or implied every cubic foot of air exhausted will be replaced by a cubic foot of outside air due to leakage ( infiltration ) from the outside