Actually i just googled my concern about how Game utilizes the CrossfireX Vram. it is nothing that VRAM of both Cards stacked rather only cores are stacked. i was totally wrong about vram stacking thing.
it is actually rendering the frames in parallel way and not in series. while having 2xGPUs, the images of application render more efficiently. and you still have 3GB vram and not 6GB. and 2x3GB = 3GB but the actions perform is became faster and faster due to parallel processing. like GPU 0 render one image and GPU 1 process second image. so minutes came down to seconds that is why you achieve higher FPS or better performance in 2x GPUS comparing to single GPU.
actually brother i did faced that same garbage or say "Artifacts Block at Bottom Right Corner" but in AMD 280X only. and i tell you the main cause is driver bug for R9 280X/2xx and 7xxx/6xxx series GPUs. According to forums 290/290x are not victim of this.
This happeing when you enable MSAA + PostFX to HIGH. actually MSAA on its own nothing to do with it, PostFX is the causing problem. just remain the MSAA @2x or 4x and
Lower the PostFX to "Normal" and you dont have them anymore. TESTED
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