Question, why would it require two displayport cables and two outputs from a video card? A displayport 1.4 cable and connection are supposed to support 5K at 60Hz.
if its the dell control board (I've got the dell) it predates the faster DP standards. It only supports 5k via dual link. Which is why a couple years ago when the newer 5k displays came out, I snapped up mine from some guy on ebay upgrading and selling it at fire sale prices (nothing like new hardware to lower the value of perfectly good old stuff).
This is correct! Though there is a newer driver board which supports USB-C (and thus one cable), but it's a lot more expensive. And since I was connecting to an 2015 iMac, wasn't needed in my case.
Because the Dell UP2715K has 10 bits per color channel, even DisplayPort 1.4 HBR3 would not be enough. It would work with Display Stream Compression, but that appears to be an optional feature of DP 1.4 that is not universally supported. Also, the compression is lossy.
Additional speculation: all 5k displays on the market so far (LG, Dell, Iiyama, ...?) were made as Apple accessories, probably with Thunderbolt ports in mind. Because the market is not that large they might share some components. TB3 only requires DP 1.2, but can carry multiple streams. The expensive LG display includes a TB3 hub that demultiplexes the DisplayPort streams and sends them off to a dual-link DP controller. Less expensive displays leave out the Thunderbolt part and require dual DisplayPort inputs for full resolution.
Right, so DP 1.3 which supports "32.4 Gbit/s with the new HBR3... a 5K display (5120 × 2880) at 60 Hz with 30 bit/px RGB color" is enough.
The UP2715K, only support dual link DP 1.2/HBR2, which was a bit of a PITA on my machine because nvidia's board partners don't always do a good job of describing which ports on the back of the card can be used for dual link.
Is that a limitation of the driver board?
https://en.wikipedia.org/wiki/DisplayPort