A Damper Unit is a manually operated fitting that can be used to control or even out the flow of air that is being drawn from two areas, with only one extractor fan used.  An extractor fan, equal tee piece and ducting can be used to draw hot stale air from two growing areas if required, but the only drawback is air will always follow the path with least resistance, so basically one line of ducting will pull the air more efficiently than the other.  By simply placing a Damper Unit into each line of ducting, it will allow you to make adjustments manually, to control the flow of air allowed through each line, and allow the air flow to be balanced and equal air removal from both growing areas achieved.