Hi,
I had similar problem after update AMD drivers to the latest one. Using mXDriver 17.10.3 and bootcampdrivers 17.12.2 the same, screen as two separate monitors.
Totally crap settings for x295m and Windows 10.
All is good with 15.301.2101-160209a-299468c whql. Only one screen in Windows preference panel with default AMD settings.