Multiple SPI Busses in Ribbon Cable
Following the logic of SPI work---with your goal, terms, and conditions---you can try this:


GND
SCK  - transits during a whole cycle, place it between two GNDs for good
NPCS - this signal is driven low (push-pull or open-drain) when active, i.e. "equals" to GND
MOSI or MISO - typically, when MOSI is active, MISO is inactive ...
MISO or MOSI - ... and vise versa (only one - MISO or MOSI - is active a time)
GND


and so on...

But, IMO, the real effect of such combining would be questionable. The proven path is to use every second wire in a ribbon cable as GND, such as Xilinx, Altera, ARM partners like Segger do.

Also, remember that at high frequencies:


a VCC could be considered as GND because there are decoupling caps between
a static signal like NPCS could be considered as VCC or GND because it's driven to one of them
a signal could be considered as static (or background) if it has no transitions while others have


Good luck.