You will not be able to get the U2515H maximum resolution 2560x1440 60Hz. The D1000 maximum using two of its video out ports is 2048x1152 60Hz.
The D1000 has VGA/HDMI/DP out ports. You will need to purchase an HDMI to HDMI cable.
XPS Laptop USB 3.0 port --> D1000 USB 3.0 cable --> D1000
D1000 DP out --> U2515H DP to mDP cable --> 1st U2515H mDP in
D1000 HDMI out --> HDMI to HDMI cable --> 2nd U2515H HDMI in