@fat16lib: Can you explain this timing a bit? At what point does the Arduino become SPI slave, and the SD card become SPI master? If the SD card is always a slave, it needs to keep quiet until the SPI master tells it to talk.
Read the SD spec - it is impossible to access two SD cards in parallel on one SPI bus. Cards are not synchronous devices. They go ready at random times and return status at random times.
It is not as much the SD spec that concerns me here. It is the SPI spec. Does the SD card transmit with the Slave Select HIGH? I don't see an interrupt pin. It doesn't interfere with the w5100 on the same SPI bus. If what you imply is true, they should not be able to coexist. ??
Maybe it is just me, but if the SPI is not synchronous, what is the SCK pin for?