U3415W Setup Extended Desktop (2 screens) with Surface Book

I am having issues being able to extend the desktop to 2 external screens. I just purchased a surface book 2 - 15" with the following specs: - 16GB RAM 1866Mhz LPDDR3 - 8th Gen Intel® Core™ i7-8650U quad-core processor - 4.2GHz Max Turbo and NVIDIA® GeForce® GTX 1060 discrete GPU with 6GB GDDR5 graphics memory. I also purchased a surface dock. The surface dock has 2 x mini DP ports and 2 x USB 3.0 ports. I also have a Dell Ultra Sharp 34” Curved Ultrawide Monitor (U3415W). The Dell U3415W monitor has the following video inputs: 1 x mini DP, 1 x DP, 1 x HDMI and 1 x MHL. And has the capability of displaying 4 discrete video inputs simultaneously (supposedly). I connected the surface dock to the monitor with 2 cables. The first connection is from the surface dock mini DP port to the monitor's HDMI video input using a mini DP to DP cable and a DP to HDMI converter. The second connection is from the surface dock mini DP port to the monitor’s DP video input using a mini DP to DP cable. When I try to extend the desktop to utilize the monitor as 2 independent screens, Windows 10 only recognizes one of the screens (the HDMI input on the monitor). Windows 10 does not recognize the screen via the DP input on the monitor. Also, the screen displays the HDMI input, but the monitor tells me there is no signal from the DP input (although it knows there is a cable plugged into the DP input). Can anyone help me? I would like to leave the surface book connected to the surface dock (with the surface book closed) and have the desktop extended to 2 screens on the single monitor. Thanks for any help you can provide.