The ability to connect a second display to the Thunderbolt 3 docking station's second (also known as 'downstream') Thunderbolt 3 port is ultimately determined by the design internal decisions made by the host system's manufacturer. 

To expand further, system manufacturers have discretion when designing their Thunderbolt 3 systems in regard to how may displays are supported via a single Thunderbolt 3 connection to the host.   Some manufacturer's support two displays via a single Thunderbolt 3 connection, while others may only support one display via a single Thunderbolt 3 connection.  We have produced a video that provides for more information on this topic for those who are curious here --> Link

Unfortunately, system manufacturer's specifications often do not include this level of detail.  The best practice is to check directly with your system manufacturer to determine the host system's capabilities.