With reference to the Z80-MBC2 retro computer's SD card module: Is it possible to hook it up for dual access by let's say a Pi Zero. So my idea is that if I could do a little hack on the SD card reader module, so I could access the SD card via a Pi Zero to read and write files. This would allow me to do all sort of extended operations such as editing and the likes. Does anyone have any idea how possible this idea is and how to go about it?
No, for several reasons:
Both the Z80-MBC2 and the Raspberry Pi assume some state of the filesystem. If one board changes it, how will the other know?
While an SPI bus (the lowest level of access for SD cards) can have multiple Chip Select lines for different devices ("one to many"), it can't have multiple data in/out lines going to the same device ("many to one") or multiple clock signals.
The Z80 requires a card reader with a data level shifter to go from its standard 5 V logic to the SD Card's 3.3 V logic. A Raspberry Pi uses 3.3 V logic directly. While there may be level shifters that might allow the two to be connected at once, this may be overly elaborate
Finally, a CP/M filesystem is not typically something that Linux can read/write without some user software doing the translation. CP/M typically doesn't even know the exact length of a file without reading it. You can ease these protocol problems, but not without lots of work.