In a comical turn of events, while NVIDIA restored NVLink SLI support for its RTX 2070 Super graphics card on virtue of being based on the "TU104" silicon, AMD did the opposite with "Navi." The Radeon RX 5700 XT and RX 5700 lack support for AMD CrossFire. If you put two of these cards on, say, a swanky X570 motherboard that splits PCIe gen 4.0 to two x8 slots with bandwidths comparable to PCIe gen 3.0 x16; you won't see an option to enable CrossFire. AMD, responding to our question on CrossFire compatibility clarified that AMD dropped CrossFire support for "Navi" in favor of DirectX 12 and Vulkan "explicit" multi-GPU mode. The older "implicit" multi-GPU mode - CrossFire - used by DirectX 11, DirectX 9, and OpenGL games is not supported. The AMD statement follows.

Radeon RX 5700 Series GPU's support CrossFire in 'Explicit' multi-GPU mode when running a DX12 or Vulkan game that supports multiple GPU's. The older 'implicit' mode used by legacy DX9/11/OpenGL titles is not supported.