We have not been tested this configuration yet, however; Dell UP2715K monitor Dell UltraSharp 27 Ultra HD 5K Monitor with PremierColor - UP2715K uses a Multi-Stream Transport System that requires 2 mDP 1.2 to actually get the full 5K resolution @ 60 Hz. I checked the requirements and you just need to plug the 2 mDP. You can check with Dell if there is any other requirement need it. What resolution do you get using only one mDP connected?
5120 x 2880 at 60 Hz (Dual DP cable required)
3840 x 2160 at 60Hz (single DP cable required)