To drive two external displays from a dock in a first-class, hardware-supported way, you need
- A Macintosh with a Thunderbolt port that supports two or more external displays
- A Thunderbolt dock, as opposed to a plain USB-C or USB-A hub or dock
A plain USB-C hub or dock that provides multiple places to attach monitors will be using either
- DisplayPort MST - which Macs do not support, or
- A second-class workaround that requires installation of a special driver, or
- Both
When you plug a dock that uses MST into a Mac, the typical symptom is that the Mac detects only one monitor – and the monitors appear to "mirror" each other. Basically, the Mac is sending out a single video signal … and the dock is simply feeding a copy of that signal to both monitors, thus the "mirroring".
With a Thunderbolt dock, a Mac that supports multiple monitors is typically willing to send up to two video signals, which the dock then redistributes to the attached monitors.
If you tell us which particular MacBook Air she has, we can use that to determine what types of expansion ports it has, and what its capabilities are when it comes to driving external displays.
