I am recently trying to find a dock which works with a DELL Precision 7540 and MacBook Pro 16" as well, at the same time. I am more or less forced to run with WD19DC/DCS, since the Dell Notebook requires more than 200W power supply to connect the dock properly (and thus the annoying dual connector). So here is my question:
Are you able to use the WD19DC with your macbook with a single Thunderbolt connector at all? I am facing the problems you describe in 100% of all cases when i try to connect only one of the two USB-C connectors. My external screen only starts working when i plug in both cables shortly after each other. While the dock itself starts working in this case, i can only connect external displays using HDMI. Trying to use DisplayPort leads to the same "laggy" behaviour without the display working.
Honestly i am kind of lost in the jungle of display protocol standards and can't find the reason for this strange behaviour. I would expect to be able to use my MacBook with a single connector since the power supply and protocols should be compatible and only be required to use the second connector with the energy hungry monster Precision 7540.
While i was writing this answer i tried to connect to the dock using only 1 usb connector with monitor connected via display port and it started to work - so at least it looks like i can confirm the behaviour you are facing. I must admit its completely annoying and disappointing having to find working solutions for hardware being as expensive as these.
The last firmware update for the dock contains the following changes:
Right now i have no solution, but i will try flash the update and keep you guys posted.
The success rate of connecting the the external monitor using since usb c is 1 out of 10 try only. I have only hdmi support on my monitor, so couldn't really try the DP port.
I also try to connect two USB C simultaneously. But it failed to detect. Can you please explain the way you connecting the two USB C cable from your Dock?
Recently I also noticed that the mac is detecting my external monitor when connected to top USB C. The one near to hinge. I can see the monitor name from the settings. But the signal not coming to the display. But if I connect to bottom USB C it won't show anything at all.
Due to this issues, I am using an USB c adapter just to connect my external display and using my docking station to connect other accessories (NAS, Keyboard, mouse etc..)
I also tried my other dell docking station D6000 which has the display link technology. But again no luck. Its not detecting the display but other accessories are fine.
I have a work laptop which connects to the dell docking station properly. So I am looking to connect my macbook pro also to the same docking station. So that I don't want to reconnect the display all the time..
If I am correct, I understood that you are successfully able to connect the external display via hdmi if you use two usb c cables from the WD19DC docking station. Could you please explain this to me? I am happy to use this way.
To answer your question, I couldn't successfully connect my external display via hdmi using single USB C or two USB C.
Hope this helps.
Thanks,