So it looks like my “new” MacBook Pro 2016 with TouchBar and Radeon Pro 460 with 4 MB does not support DisplayPort Multi-Stream Transport (MST). MST allows the daisy chaining of supported monitors.
I got a USB-C to DisplayPort cable from Amazon and connected it to my first Dell UltraSharp U2515H.
The display finally mirrored my MBP, but I had to move USB-C ports seemingly to “wake up” the monitor or MBP. Then I activated the DP1.2 mode on the U2515H since by default DP1.1a is the factory setting.
Then I connnected the video out of the first UltraSharp to the video in of the second UltraSharp. For this, I used the Dell-provided DisplayPort to mini-Display Port cable. I then activated DP1.2 on the mDP port on the second monitor (not sure if I had to do this but thought it made sense).
The second UltraSharp came to life and displayed a mirror of the first UltraSharp. My laptop screen showed one desktop while both UltraSharps displayed a second (mirrored) desktop. The MBP recognized one external monitor, not both.
I also tried using a DP-to-DP cable to interconnect the two DP1.2-compatible monitors together, but that didn’t change the results (yes, I activated DP1.2 for the monitor’s DisplayPort). My MBP is running macOS Sierra 10.12.5.
Now the Dell manual states that only a single external display with resolution of 2560×1440/60Hz is supported by MST, and that’s the native resolution of the Delll UltraSharp U2515H. So there’s that. I tried reducing the resolution of the Ultrasharps seeing if that had any effect, but nope.
So either MST isn’t supported/implemented in the Mac OS X software or Mac hardware or I need to try this again using monitors with lower resolutions. Or most likely, I need to forget MST and go the old fashion route of using two video connections to two monitors.