I was starting to look into doing this until I decided to use a Pi Pico so I could move it from machine to machine (just plugging into the USB of the host Pi).
I could be totally wrong as I haven’t done this yet, but as it was explained to me, Klipper determines which SPI bus to use based on which CS pin is specified. If that’s true, you need to specify the associated CS pin for each of them. If I understand correctly, you don’t want the spi_bus line with that config as it is for software SPI (and I assume you want hardware SPI since you’re not calling out each pin).
Unfortunately, it doesn’t look like GPIO18 is related to SPI0 or SPI1.
I’d recommend using GPIO8/SPI0_CS0 for SPI0 and GPIO16/SPI1_CS0 for SPI1.
Here’s the pinout I’m referencing:
