This package can be used to either exhaust double the amount of air or to setup a push/pull configuration.This would involve using one unit to pull air out (exhaust) and the other to push air in (intake). If using this configuration, the exhaust vent should be mounted high while the intake vent is mounted lower in the cabinet. It is usually preferable to have both vents pulling air out as fresh air will come in through even the smallest of openings around doors or through wiring holes, etc.