MBP13 + Lenovo TB3 doc only mirrors dual displays
 

I have a MPB13 connected to a Lenovo TB3 doc (model AC40). MBP is running Catalina and System Information reports DisplayLink version 5.2(54) is installed and Loaded. Dock is connected to MBP using a Toshiba active TB3 cable. Enet, audio, etc all connected via. the dock and appear to be working.

I have two 24" monitors (Samsung LS24H850QFNXZA). Each monitor connected to one of the DisplayPort ports on the dock. HDMI/VGA ports are not connected.

SystemInformation->Graphics/Displays only reports one of the monitors, however both monitors are being driven in mirror mode.

I've read through much of what Google has to say and it seems like this configuration should be supported, although most of the posts don't specify which Lenovo TB3 dock is being used (AC40 vs. gen2, etc).

Any thoughts on how to get my 2 monitors working in extended mode using the 2 DP ports on the dock?

thanks
Mark

Update: These monitors have usb-c input in addition to DP. If I connect one of the monitors to the AC40 dock via the dock TB port, I can drive them in extended mode. I think I saw that as a solution, but wondering if anyone knows why I can't use the 2nd dock DP port to drive the the second monitor. I don't really want to burn the front TB port to drive a monitor when I should have enough connectivity on the back ...

According to the information provided by Lenovo here --> https://support.lenovo.com/us/en/solutions/acc100356 the docking station you are using does not use DisplayLink technology, so DisplayLink will not be able to assist.

While not called out explicitly by Lenovo, it sounds like this dock has an internal DisplayPort MST hub within the unit that enables multiple display output. DisplayPort MST setups are not supported by macOS systems. When a DisplayPort MST device is used with a Mac the symptom is that the displays work, but all of them are in mirror mode and recognized as one display.