S3423DWC, issue with PBP and WD19S

Hi all,

I'm having issues with a new S3423DWC we bought and setting up PBP. I'm using it to connect to a Dell WD19S 180W docking station using the HDMI1 and display port (USB-C) cable. I've set up PBP in the monitor setting, however I'm only seeing one of the inputs?

I've got a second monitor, Dell 27" C2722DE also connected to the dock using DisplayPort cable which is working fine. 

Is this a power issue thing that I can't run PBP on the S3423DWC using HDMI and the USB-C cable to the dock? If I replace the USB-C cable with an HDMI and run it straight to the laptop, it works fine. I can't connect both HDMI1/2 to the dock as only 1 available HDMI port.

A crude overview of the architecture is below for reference.