MBP 2017 / Mojave 10.14 / DL 5.2.1 - Can't extend monitors
 

I have a MBP as noted in the subject line connected to a Lenovo 40A9 dock.

When I use the dock with my Lenovo laptop the laptop and the dock drive my two external DP monitors fine in extend mode. I actually get three (laptop +2)

When I connect my MBP, I an only mirror the two external monitors. I have seen a lot of google results that say the this OS/Driver combo should have enabled extend but I can't seem to figure it out.

I did the driver install and the two steps of permissions. I checked my system report and the driver is enabled in the stem report.

Am I missing something? Both external monitors are duplicates of each other.

Clarification... I can extend from my MBP to the external monitors but both external monitors are exactly the same.

Hello,

I do not believe this docking station uses DisplayLink technology.
Therefore, using the driver will not affect your displays through that product. I don't believe the product you have can do what you're looking for.
The Lenovo current dock with DisplayLink is the Lenovo Hybrid USB-C USB-A which can achieve what you want to do.

Kind regards,
Alban

That dock does support video over USB-C as it works with my Lenovo to the two display ports already. Not sure the language that is uses but DisplayLink over USB-C is a thing.

And it is displaying now from my MBP. Just in mirror rather than extend.

Yes, and you will not be able to extend with that product on the mac.
DisplayLink is different from DisplayPort Alternate Mode.

Does anyone know the technical reason that Win10 laptops with a Thunderbolt3 output can use this dock to extend two external monitors while OSX can't?

Or secondarily, what is the technical reason OSX CAN display to two in mirror mode but not in extend mode?

My impression based on the specifications of the dock (https://support.lenovo.com/us/en/solutions/acc100348) is that the unit has an internal DisplayPort MST hub inside the unit in order to facilitate the multiple display support (note that 'DisplayPort' is not the same as 'DisplayLink').

DisplayPort MST hubs are not supported in macOS. When such devices are connected to a macOS system, the behavior is that all displays connected to the DisplayPort MST hub will 'mirror' each other, as you describe.

In short, the problem appears to be the internal limitation of macOS.