Well 2x 6900xt do have the mGPU option, but it has own issue...I wont suggest people try it cos it is painful. im runing 2 6900xt rn and im still working on how to make them work. Dx12 or Vulkan dont = mGPU support, so u will have a hard time. Amd removed crossfire support for 6000 gpus so u cant even play crossfire games...
TRUST ME THERE ARE NOT ENOUGH SUPPORT FOR THIS! AMD/MICROSOFT KNOW NOTHING ABOUT THIS. IF U DONT BELIEVE ME JUST TALK/EMAIL THEM, IM SURE U WILL BE BACK.