MergerFS Multidisk Mount Batocera
Make required directories mkdir -p /userdata/custom/bin && cd /userdata/custom/bin/
Download newest static mergerfs build, current built now can be found HERE
extract tar -xvzf FLIE_NAME.tgz
Move the mergerfs bin to /userdata/custom/bin and delete the other stuff
Batocera will mount some disks and not others, not sure of how they do, but just check to make sure all of the disks you want are mounted and if not add the mountpoints to the script below