RPI Companion Board

New user, and ambitious student here.

In my designed drone setup I have an onboard Raspberry Pi companion board which gives the Pixhawk autopilot waypoints, and other flight commands, additionally I want to connect the RPI to a LoRa shield, this is where I have become unsure on how exactly this will work.

I have found some documentation on how i can use SPI to control if the LoRa board receives data from the RPI, but i have been unable to determine how I set this up with the pixhawk.

Note: The information I have found so far has been about having the pixhawk as the SPI master with multiple SPI slaves, but here in my case the RPI is the SPI master and I have two SPI slaves, the LoRa board, and the pixhawk.

Would love to hear advice on how to set this up, and any links to relevant pages, documentation.