Pretty sure the DX11 client isn't in any of the SLI/CF profiles. At least on the AMD side, Crossfire hasn't worked properly for FFXIV in years. Wouldn't be surprised if the situation is similar for Nvidia cards. Try disabling SLI. I also know that the DX11 mode can be very buggy when running in Borderless Window, so maybe try real Fullscreen if you haven't already (though if that's the case, SLI will be disabled anyway since multi-GPU solutions only work in fullscreen).