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The Plugable Thunderbolt 3 Dual DisplayPort Adapter provides a solution for a PC to connect to two 4K@60hz displays using DisplayPort technology. But since this only applies to Thunderbolt 3 PC's which have routed 8 lanes of 5.4 Gb/s HBR2 DisplayPort signals to be tunneled through the Thunderbolt link, both the Spectre x360 (2nd Generation 2017) and HP Spectre Laptop (13.3") don't seem to support this.

To demonstrate this, first we connected the LG UltraFine 5K Display and an LG 31-inch Digital Cinema 4K 31MU97 Monitor using standard DisplayPort to standard DisplayPort cables between the Plugable Thunderbolt 3 Dual DisplayPort Adapter and the monitors.

When connecting the Plugable Thunderbolt 3 Dual DisplayPort Adapter to the Spectre x360 (2nd Generation 2017), we accepted adding the new Thunderbolt device in the pop up even though external displays connected through the Plugable adapter still work regardless. If you don't want to keep on seeing the 'New Thunderbolt devices have been attached' pop-up, select 'Always Connect'.

In the Windows System Tray there is also a Thunderbolt icon which if you select, will show the Plugable Thunderbolt 3 Dual DisplayPort Adapter  enumerated with the Spectre x360 (2nd Generation 2017)'s Thunderbolt controller.

A more convenient way to set up your 4K displays on the Spectre x360 (2nd Generation 2017) is to go to the Intel HD Graphics Control Panel. 

When you first connect the Plugable Thunderbolt 3 Dual DisplayPort Adapter to the Spectre x360 (2nd Generation 2017), you will immediately notice that only either the LG UltraFine 5K Display or the LG 31-inch Digital Cinema 4K 31MU97 Monitor will turn on at one time with the Spectre x360 (2nd Generation 2017). 

You will need to select the Multiple Displays screen to see both the LG UltraFine 5K Display and the LG 31-inch Digital Cinema 4K 31MU97 Monitor  being set up as Active Displays.

Unfortunately, only the LG UltraFine 5K Display enumerated properly at 4K (3840 X 2160) @ 60hz while the other monitor was not able to support 4K@60hz display at the same time. We could also confirm that the LG UltraFine 5K Display could only achieve 4K (3840 X 2160) @ 60hz resolution due to the limitations of the internal Intel GPU used by the Spectre x360 (2nd Generation 2017).

When we replaced the Spectre x360 (2nd Generation 2017) with the HP Spectre Laptop (13.3") over the same Thunderbolt port, the Plugable Thunderbolt 3 Dual DisplayPort Adapter could also be seen to enumerate properly with the HP Spectre Laptop (13.3") but only 1 external 4K@60hz display is supported.