I've been having this issue for a while now and was assuming that AMD would fix this with usual updates.
I have 2 LG 25" Ultra-Wide 21:9 monitors and I cannot connect more than one monitor at a time to my Sapphire R9 290X 8GB. The other monitor does not appear in the Display Settings for Windows 10. Prior to my upgrade to Windows 10, I was running Windows 7 which had no issues with these two monitors. I am running 1 monitor on HDMI (the one that is currently working properly) and monitor on Display Port (which is currently not working at all). I have used this monitor with success on another device other than my computer (an Xbox 360) using the HDMI input. I am not sure if this is just an issue with Display Port on my card or something larger.