I am running Windows 10 Pro N with two Sapphire Radeon R9 290X 4GB GDDR5 DUAL DVI-D/HDMI/DP PCI-Express BF4 Edition Graphics Card (21226-00-53G) cards.
Each has a LG 23EA63V-P monitor plugged in via HDMI.
One of the monitors is not detected (does not show up to extend or in hardware manager). I have tried different versions of the driver packages with and without Catalyst / Radeon controls.
The monitor and HDMI Cable work when swapped to the other GPU. I do not have crossfire enabled.
When installing the drivers it briefly identifies the monitor and tries to extend the monitor, but fails to detect again when they complete.
This worked flawlessly in Windows 7.
Are they both detected in device manager? Sounds like an incompatibility issue with Win 10.