P2415Q, two, TB16, Latitude 5480, incompatibility

@dan2021  You don't have a good solution available to you here.  You have varying degrees of bad solutions.

The TB16 requires Thunderbolt 3 support on the system side.  If the system doesn't have it, then it can't use the dock at all, as you've found.

Dell's newer WD19TB dock uses a newer Thunderbolt chipset that supports backward compatibility with regular USB-C, non-Thunderbolt devices, but it operates in reduced functionality in that mode -- specifically, a functional level equivalent to the regular WD19, which when paired with the Latitude 5480 would also be functionally equivalent to the WD15 you were using previously.  So the WD19TB would work optimally with your XPS 15, including providing adequate power and running dual 4K 60 Hz, and it would work to some degree with the Latitude 5480, but not enough to run dual 4K 60 Hz, or for that matter even dual 4K 30 Hz.

The only dock that could run dual 4K 60 Hz from both of those systems would be the Dell D6000.  However, that dock can achieve this because it relies on "indirect display" technology called DisplayLink -- not to be confused with DisplayPort.  That comes with a lot of potential drawbacks that I've written about in the post marked as the answer in this thread.  And on top of that, it only supplies up to 60W over USB-C, which isn't enough to run the XPS 15 properly, so you'd have to keep that system's power adapter connected separately if you used that dock.

Bottom line: If you want to run dual 4K 60 Hz through a docking station using native GPU outputs rather than hacks like DisplayLink, you need systems that support Thunderbolt.

@jphughan 

"If you use a male USB-C to female DisplayPort adapter dongle and then a DisplayPort cable from that dongle to one of your displays, then you can just move that DP cable over to the other DP output of the dock when your Latitude is connected.  If on the other hand you get a  USB-C to DP cable as a single entity, then you could move that cable between the downstream TB3 port (for the XPS) and the USB-C output near the HDMI output (for the Latitude) as needed."

I don't mind switching a cable everytime I switch laptops. Based on the above, which method do you suggest?

@dan2021  You'll be limited to 1920x1080 when running from the Latitude.  In terms of image quality, each pixel in a 1920x1080 image would map perfectly onto a 2x2 physical pixel grid on your 4K displays, without any scaling artifacts, which is handy.  And since your displays are 24", what you'll end up with is a 24" 1080p display experience.  24" 1080p is probably the most common PC external display on the market these days, so it should give you a very typical PC experience.  Just keep in mind that you may have to adjust your resolutions using the Intel Graphics Control Panel / Command Center rather than Windows in order to get the system to force the active signal resolution down, rather than just the desktop (rendered) resolution.  The former is important because if you don't change the active signal resolution, you won't be changing the bandwidth requirements of your setup, which you have to do in order to get the non-Thunderbolt system to work properly with those displays given that it can't push nearly as much bandwidth to the dock as the XPS 15 will be able to.

@dan2021  If you've already got a DP cable for that display and a USB-C to DP dongle is cheaper or easier to find than a USB-C to DP cable, then go with the adapter.  Otherwise, go with the cable.  The end result is the same.

@jphughan 

Thanks again for the detailed explanation.  It looks buying the WD19TB dock will the best (of the worst) solutions.  I certainly don't need true 4K on my work laptop, so can accept some degree of image degradation in order to accomodate the Latitude system.

Do you know how how much screen quality or resolution I will lose on the Latitude via this setup?

Thanks!

@dan2021  I already discussed the D6000 in a previous reply in this thread. See above.

@dan2021  The WD19 130W can pass up to 90W through to the attached system, while the WD19 180W can pass up to 130W through to the attached system.  If you won't be using any systems that are designed to have a 130W power source, such as XPS 15 or Precision 5000 Series models, then the 180W version would be a waste of money.  The WD19TB only comes with a 180W power supply for 130W passthrough whether you need it or not.

@dan2021  Unfortunately as it seems you already discovered, the regular WD19 won't work for dual 4K displays.  That requires 4x more video bandwidth than the regular WD19 is able to access from the XPS 15 9560.  The WD19TB can access that much bandwidth, but not the regular WD19.  And again, in order to run dual 4K 60 Hz from that system, you'll need to connect one of the displays to the downstream TB3 port to split the bandwidth requirements of the displays according to how the dock divides available bandwidth from the system across its ports.  And then you'll likely need to move that display to another output when you use the 5480, in addition to using those displays at 1080p from that system.

I read another thread and realized I need the WD19TB!

@jphughan 

The WD19TB arrived, and I've slotted it in, and from a picture perspective everything is great.  Dual 4k picture when plugged into my XPS.  I have one monitor direct into the DP port, and the other into the TB port via a DP to USC-C dongle.

Now two new problems have manifested:

1. I cannot update the firmware as when I try running it, get an error that says "No WD19 attached. This firmware update utility only works with a single WD19 attached."  (mind you I did install this firmware last week with my WD19)

2. After updating the Realtek USB Audio driver, the system is not recognizing any other sound sources such as the monitor inputs or my external speakers. It only recognizes the XPS onboard sound.

Pls help, thanks!