The Intel NUC6i7KYK NUC Kit is the first NUC to support Thunderbolt 3 technology which among other things enables dual 4K@60hz displays as long as this is supported by the PC as well.
When we tried using the Plugable Thunderbolt 3 Dual DisplayPort Adapter with the Intel NUC6i7KYK NUC Kit, we found that it could not connect to dual 4K@60hz displays.
First we inserted the Plugable Thunderbolt 3 Dual DisplayPort Adapter into the NUC kit, and confirmed the attached Thunderbolt device.
From the Screen Resolution window, we could only see one of the 4K displays that was connected to the Plugable Thunderbolt 3 Dual DisplayPort Adapter at a time.
Unfortunately, the Intel NUC6i7KYK NUC Kit does not support two 4K@60hz displays using the Plugable Thunderbolt 3 Dual DisplayPort Adapter.