Shower in one bathroom causing moisture in the second
If both fans are venting out the same exhaust port, i would check you have back-draft Dampers (one on each section before the presumed Y connector).



Its possible your first extractor is just blowing the damp air into the second bathroom - a damper should add enough resistance to the flow of air to make the damp air "prefer" exiting via the exhaust port rather than going through the second damper (and in to the second bathroom, causing the damp), without restricting airflow noticeably.