[NAME]  by the way, one more thing in addition to the above.  Speaking of manuals, I'm pretty sure that if you check the P2415Q's manual, you'll find that if you enable MST, the display will limit itself to 30 Hz.  That's because again, the current DisplayPort revision at the time those displays were created didn't offer enough bandwidth to run 4K 60 Hz with enough bandwidth left over to pass through to another display via MST, so the design assumption made on those displays was that if you chose to enable MST on that display in order to set up a daisy chain, then you'd actually want enough bandwidth left over to pass through to the secondary display you'd be connecting, and thus the displays would limit themselves to only 30 Hz to cut their bandwidth requirements in half.So disable MST on both of your displays, connect one of them via the "upstream" Thunderbolt 3 port on the WD19TB using your USB-C to DP cable, and use your DP to mDP cable to connect the second display to one of the DisplayPort outputs on the WD19TB.
How do I get 60Hz to both of my P2415s running at 4K? I'm running Win10 with the latest updates installed. Primary monitor is connected from the WD19TB's TB/USB-C port using a USB-C to DisplayPort TB3 compatible cable to DP (monitor set as MST Primary) which is in turn daisy chained to secondary monitor using a DP to mDP cable (monitor set as MST secondary). Everything is working fine at 4K 30Hz -- but I want 60Hz!
XPS 9570 with WD19TB and 2xP2415Q -- no 60Hz @ 4K?