While I wait on this to be resolved, the remaining insurmountable problem (for me) is that I cannot use the Ethernet Featherwing and an SD Feather card together. The code locks up. I tried to disable the Ethernet while I write to the SD card then re-enable it after that but this solution is not reliable.
This is a known issue and I don't believe there is a way to work around it before it's fixed.
If SPI transactions were enabled in SDFat in theory the two should not conflict with each other as there is a mutex to prevent them from using the shared SPI resource at the same time. However, the system and user code use two different mutexes, thus not preventing each other from accessing the shared resource at the same time.
@Jimmie, as @rickkas7 pointed out, this is DeviceOS issue, not a hardware issue. A de-multiplexer will not solve the problem. Also, there is already a thread that you participated in regarding this so please don't hijack this topic to continue the discussion.