For Thunderbolt 3 systems:
Some systems with Thunderbolt 3 support may only have the ability to connect a single display through the Thunderbolt 3 port. This is due to how the system manufacturer has configured the internal circuitry of the computer between the system graphics card and the system Thunderbolt 3 controller. To be specific, they have only routed one DisplayPort video line/stream from the graphics card to the TBT3 controller.
As an example of this, many Intel NUC systems only support a single display, see here for reference: https://www.intel.com/content/www/us/en/support/articles/000021752/intel-nuc.html
We also see this configuration on many high end gaming laptops or desktop replacement workstation laptops that have one or more dedicated onboard video outputs. Unfortunately, this is often undocumented and you may need to contact your system manufacture to verify if they expect dual monitors will be supported over Thunderbolt 3.
Apple M1 based Macs (MacBook Air, Mac Mini, and MacBook Pro) only support a single display through Thunderbolt 3 or USB-C. (This is a limitation of the M1 graphics card design.)
For USB-C systems:
Apple USB-C only systems (without Thunderbolt 3 support) like the 2015 MacBook Retina 12" can only support a single display or will show two screens with mirrored images. This is because Apple does not support DisplayPort MST (multi-stream) which our dock uses to take a single USB-C DisplayPort Alternate Mode video line and connect two displays through it.
Windows USB-C systems should support MST in most cases, if you're only able to connect a single display, please contact us via firstname.lastname@example.org for further assistance!