Well, it doesn’t offer better PERFORMANCE having both enabled in a hybrid system. It offers the high performance of the discrete card while still getting the lower power draw of the IGP. The level of performance of the Intel IGP even if it was possible to use them in a hybrid Crossfire configuration, the IGP wouldn’t add sufficient performance to raise the overall metric any. Given that it looks like yours is a desktop and power draw (thus battery life) probably isn’t that important, I’d disable the IGP and run only on the AMD card.
Ok so I tried what you have suggested, it does seem to detect AMD graphics card, but with one of my tests when I switched to Xanmod kernel, it actually performed better on Intel graphics rather than AMD graphics which is strange. Any idea why this is the case? (the first post was done with vanilla kernel)
But it still has this glitchy effect in Blender. I am not too sure why this is still happening?
So no idea on the blender side. For OpenArena, given that it’s getting nearly 400 fps with either, I think it comes down to the graphics being so non-taxing to the graphics subsystem that it’s being CPU constrained, not GPU, therefore the 2 results are within a mathematical rounding error of identical.
If you experience issues  with a AMD R9 390 series graphics card, set radeon.cik_support=0 radeon.si_support=0 amdgpu.cik_support=1 amdgpu.si_support=1 amdgpu.dc=1 as kernel parameters to force the use of amdgpu driver instead of radeon.
If it still does not work, try disabling DPM, by setting the kernel parameters to: radeon.cik_support=0 radeon.si_support=0 amdgpu.cik_support=1 amdgpu.si_support=1