has this issue been sorted out with High-Sierra?
The issue being (in case you didn't want to click the link) that daisy-chained displays were always being mirrored instead of recognised as individual displays.
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I have installed Windows on to a Bootcamp partition and os has no problems daisy chaining the displays. If i return to OSX i can get only mirrored display on my Dells. The problem is that Apple supports only thunderbolt dasychaining. If you are using display port it has no support (or it disabled, case and point: windows os) for MST displayport connections.
I have the same issue, both of my Dell U2515 are mirror each other. They are connected to the docking station via D-port to D-port and the other D-port to HDMI on the docking station. I have tried many thing to make them work separately but no luck. So frustrating. I have the 15-inch MacBook with touch bar and running high sierra. I was going to try getting thunderbolt-d-port cabled and connect them direct without docking station and see what happen. Thanks