Hi jjsa,
Thanks for getting in touch.
Your device is working as expected, and all in all I'm surprised Targus didn't give you further explanation other than that no additional drivers are required.
Indeed, no additional drivers are required for the Targus DOCK423A as this device is not equipped with DisplayLink technology at all which you will find confirmation of on the
product page
You can also read the following there:
* HDMI ports support USB-C DP Alt Mode and requires compatible USB-C DisplayPort Alternate Mode host. Supports (2) independent screens in Windows or Chromebook, (1) in iPadOS and (1) in macOS (or dual mirror)
Which basically means that it is expected that on macOS the two connected screens will clone each other and appear as one on the system. The reason for this is that Apple does not support MST in its computers. The solution to your problem would be to choose a device based on DisplayLink technology.
I hope you find my message helpful.