Dual 4k 60Hz with Precision 5540 and WD19DC

@Waylander63  The WD19DC is not a Thunderbolt dock; it is a "dual USB-C dock".  And it can only run dual 4K 60 Hz when used in dual connector mode.  But that mode is only supported on Precision 7000 Series systems.  When used in single connector mode, it operates like the regular WD19, which when paired with your system would only have enough bandwidth for dual 1080p displays or a single QHD display, or a single 4K 30 Hz display.  But the WD19TB would solve your problem.  It DOES support Thunderbolt, and as such it would be able to tap into 4x more display bandwidth than the WD19DC in single connector mode, enough to run dual 4K 60 Hz.  Note that with a system like yours that runs DisplayPort 1.2/HBR2 over USB-C/Thunderbolt, in order to run dual 4K 60 Hz, you'll need to connect one of the displays to the downstream TB3 port at the edge of the dock (this port doesn't exist on the WD19 or WD19DC).  This is not to be confused with the USB-C port near the HDMI output.  You can use a USB-C to DisplayPort cable to make this connection.  The other display can be connected to any other output.  This cabling requirement is mentioned in the WD19TB User Guide, and it has to do with how the dock allocates video bandwidth received from the system across its outputs, which varies based on the DisplayPort revision supported by the attached system.  With HBR2 systems, half of the total bandwidth is allocated to that downstream TB3 port, and the other half is shared across its other outputs.  So if you don't use that downstream TB3 port, you'd be trying to run dual 4K 60 Hz displays utilizing only half of the total video bandwidth available on the dock, which won't be sufficient.

@Waylander63  I forgot to mention that the WD19TB has been replaced by the WD19TBS, along with all other WD19 models that have been replaced with an "S" model.  The "S" models are identical to the non-S models except that they lack the headset jack.  Evidently something about the semiconductor shortage caused Dell to be unable to add those jacks to their docks, so they created new dock variants without them in order to be able to continue producing docks.  If you don't care about that jack, then a WD19TBS would work for you.  If you do care about it, then one option would be to buy a WD19TBS and then swap its interface module (the portion of the dock to the left of the seam) onto your WD19DC dock "body".  The documentation for these docks covers this.  That would turn your WD19DC into a WD19TB, and then moving your WD19DC's interface module onto the WD19TBS would turn that dock into a WD19DCS.

Thank you @jphughan, this explains a lot. As a matter of fact, the setup was originally planned for Precision 7000 system and then downgraded to the 5400.

Given I don't care about the headset jack, is the WD19TBS interface module available as a field-replaceable unit? Is not only a matter of saving some bucks but also to reduce electronic waste.

Thank you for your excellent and prompt support.

@jphughan​ My "dual connector mode" appears not to be working. Have been without a laptop for about a week, and the new laptop only lights up one of the two connectors. They both appear to work in isolation, so i think they're both fine. It looks like the one that it detects first is the one that lights up, because it's not always the same one. Also tried each usb-c port, same result. Tried swapping the connection and still no go. 

Got to this thread because one of my (4k) monitors isn't working at all and the other one seems to only want to run at 4k. Interesting windows detects both monitors, but no signal is getting sent to one. Sorry to post to this (probably old) thread.

@pfox72​ I'd need more information to even try to help out here. What is the exact model of your new laptop, including the model code? (Don't just say XPS 13, for example.) And what is the exact model of the dock you're using? I realize this thread was created for the Precision 5540 and WD19DC, but I don't assume that means everyone who posts in the thread will have the exact same equipment, since that's often not the case.

Second, you talk about only one connector working, which I originally interpreted to mean connections from the system to the dock, but then you said "the one it detects first is the one that lights up", which sounds like it would refer to connections from the dock to displays.