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Newer cards don't use cables they transfer the data over the PCIe bus. If both slots are running at PCIe x8 speed (at least) and the motherboard does support Crossfire it should be OK. Check in the system / hardware tab of the Radeon Settings app. Also you would need to check both GPUs for the Crossfire setting in Gaming / global settings as it isn't guaranteed to be the first card that shows the Crossfire option.
Thanks a lot! It makes sense to me, the issue is resolved. X8DAH has 3 physical x16 slots and they work at x16, x8, x16 (PCIe 2.0 )
I inserted cards on both x16 slots, now I insert them to x8 and x16 slots. This doesn't fully follow your suggestion, anyway, the Radeon setting
now show crossfire enabler!. I needed to insert the 1st card on x8 slot for this case.
An benchmark (3D11) indicates it works fine (12,000@one, 16,000@cf in HD resolution).
Thanks again for your great suggestion!