“Boot Camp Unified Driver for Windows 10” Adrenalin 18.10 has no issue with split display on Radeon Pro 580, but the framerate drops a little bit as if V-Sync is enabled.
Though Radeon Software Version is 17.12, components are almost the same as Adrenalin 18.10.1.
For those curious about the components installed by AMD's new Boot Camp Unified Driver for Windows 10 v18.10, here they are:
AMD Radeon Software 17.12
Driver Packaging Version - 18.10.16.09-180628a-331348C
Radeon Settings Version - 2018.0628.1823.31270
2D Driver Version - 8.1.1.1634
Direct3D Version - 9.14.10.01334
OpenGL Version - 6.14.10.13521
AMD Display Driver/OpenCL Version - 24.20.11016.9001
AMD Mantle Version - 9.1.10.0254
AMD Mantle API Version - 102400
AMD Audio Driver Version - 10.0.1.6
Vulkan Driver Version - 2.0.20
Vulkan API Version - 1.1.70
Curiously, this is the first time (as far as I can recall) that AMD has released a Boot Camp graphics driver that had not previously appeared in Apple Software Update.