Shared or separate SPI buses for peripherals in a PCB?
1 - If your SPI clock is slow, less than 10 MHz, you can use a GPIO to generate the clock. At this speed, you have enough timing margin, you don't need a dedicated clock output. The SPI "Clock" can be generated using a counter.

2 - Having a dedicated SPI clock is usually better for EMC. I have a seen a few cases where a slow-ish free-running clock would cause noise and EMC problems.

3 - Having seperate SPI links mean you can communicate in parallel with these devices. It can be helpful in some applications...