I had this exact same issue with Dell U2717D monitors. The monitors were mirrored and showed up in the display manager as one single monitor. I played with enabling/disabling DP 1.2 on the second monitor to no effect. Finally I found out that there was another setting in monitor configuration that was simply called "MST". Evidently this is called "Multi-Stream Transport". Once I ENABLED MST (in addition to DP 1.2) on the first monitor, the displays un-mirrored themselves and Windows treated them as separate monitors. Enabling DP 1.2 and MST on the second monitor allowed me to daisy-chain a third monitor. They are all recognized as 3 separate monitors and can have independent resolution settings. At least for me, MST was the key. The MST setting on the U2717D is under DISPLAY -> MST. For me, at least, this fixed the issue of the displays mirroring while daisy-chained.