Low Power SPI Module communication between Microcontrollers
It does not matter which SPI module you use, so you can connect SPI0 of one microcontroller to SPI1 of the other. The SPI modules (like any others) are independent of each other.
The main reason there are multiple SPI's is to connect two devices (each with their own SPI controller), or to have a more flexible configuration. Mostly pins are having multiple functions, and by using SPI0, maybe UART1 does not work as they share pins, so you can use SPI1.
For the chip select pins, if you use SPI1, you should use CS1. On some microcontrollers I thought they can be any pin, but with the ones you use it seems to be hard-wired.