TM4C1290NCZAD: Also TM4C1231 SSI interface as SPI or QSPI interface w/ FSS as Chip Select..

Newman said:

For the TI SSI interface (for the TM4C1231 device, and  TM4C1290 device specifically).  We are using this SSI interface as SPI or QSPI interface.  Should we use the respective FSS signal for the SSI interface for Chip Select?  What happens if we DO NOT use it?

If you do not want to use FSS which acts as a SPI chip select signal then you can use a GPIO pin for chip select. 

Newman said:

Say we need to connect to two SPI device with the same CLK/Data pins, meaning we will need to have 2 Chip select.  Can I use one dedicated FSS signal for one device, and another one using a GPIO as a CS?  Or should I use the two dedicated FSS signal for each device (each SSI interface seems to have 2 different pins as FSS)?

I will suggest you use two GPIO pins instead. Although each SSI interface has 2 different pinouts for FSS but they will be active at the same time. This will be selecting both SPI devices at the same time. This is not what you want. Using two GPIO pins, you can control which SPI device to talk to.