A few days ago my Dell P2715q developed a line of dead pixels down one side of the screen. That was unfortunate, but Dell support sent me out a replacement display. I received it today, so while I had two working displays in my possession, I hooked them both up to verify that I could get them both working in case I want to add a second in the future. By creating another custom resolution in the Intel graphics customizer, I was able to drive both displays at 50hz - one connected to the dock, one connected directly to the surface. Poking around in the monitor menus, I found an option to enable MST. Connecting one monitor to the Dock, and then daisy-chaining the second display off the first, then enabling MST, I was able to drive both monitors off the single dock port at 30hz. The question that then leaves me is, why can I run two 4k monitors off one cable at 30hz, but not a single 4k monitor off that cable at 60hz? I don't yet understand enough about how this works to answer that question yet. Anyhow, thought it would be useful to post here about my experience. If anyone has anything they'd really like me to try, I'll probably have both of them for another day or two.