For data loopback, the I/O signals of the two controllers are connected together: the clock, slave select, MISO, and MOSI signals from one controller are connected to the other controller’s clock, slave, MISO, and MOSI signals, respectively. This connection is internal to the controller and does not use any MIO pins.
The loopback mode is selected by setting the MIO_Bank2_Loopback [SPI0_LOOP_SPI1] bit = 1.