Same Laptop, connected to a TB16, drives the same monitors to 4K using the HDMI and Mini-DP port with those two ports conecting to the HDMI ports on my identical 4K Samsung monitors. When I swap in the WD19TB and connect the WD19TB DVI ports to the DVI ports on the identical monitors, the middle DVI port drives Monitor one to 4K but the left most port (viewed from the back) drives the second 4K monitor to 2K only.
@Anonymous The ports on the WD19TB are DisplayPort, not DVI. But the answer to your question is that the WD19TB allocates video bandwidth from the system across its ports in a specific way, and that way varies based on the DisplayPort revision supported by the attached system. The Precision 5540 is a DisplayPort 1.2/HBR2 system. As noted in the WD19TB User Guide on support.dell.com, when paired with a Thunderbolt-capable DP 1.2/HBR2 system, if you want to run dual 4K 60 Hz displays, one of them needs to be connected to the "downstream TB3" port at the edge of the dock -- not to be confused with the USB-C port near the HDMI output. Use a USB-C to DisplayPort cable/adapter to make this connection if needed. The other 4K 60 Hz display can be connected to any other output. Basically, half of the video bandwidth coming from the system is allocated to that downstream TB3 port, and the other half if split across the remaining ports. In your current setup, you're trying to run your two 4K 60 Hz displays using only half of the total video bandwidth coming into the dock, and that isn't enough for dual 4K 60 Hz.
And on a side note, I'd really recommend against using "2K" when talking about resolution because so many people wrongly use it to refer to 2560x1440 resolution. 2K is technically a film resolution of 2048x1080, but when used in the consumer space it refers to the very similar but slightly LOWER resolution of 1920x1080 -- just like 4K refers to a film resolution of 4096x2160 but in the consumer space refers to a lower resolution of 3840x2160. But the fact that so many people use 2K incorrectly means that whenever you see it, you can't be sure what the person means. The resolution of 2560x1440 is rightfully called QHD or 1440p. "QHD" is only one more character to type than "2K", but it adds a lot of disambiguation value -- and disambiguation is useful when having technical discussions. 🙂