unionfs-fuse /changedata=RW:/immutedata=RO -o cow /data

/data -fstype=fuse,cow /changedata=RW:/immutedata=RO

handle_packet: type = 5
handle_packet_missing_direct: token 19, name /data, request pid 6063
attempting to mount entry /data
lookup_mount: lookup(file): looking up /data
lookup_mount: lookup(file): /data -> -fstype=fuse,cow /changedata=RW:/immutedata=RO
parse_mount: parse(sun): expanded entry: -fstype=fuse,cow /changedata=RW:/immutedata=RO
parse_mount: parse(sun): gathered options: fstype=fuse,cow
parse_mount: parse(sun): dequote("/changedata=RW:/immutedata=RO") -> /changedata=RW:/immutedata=RO
parse_mapent: parse(sun): gathered options: fstype=fuse,cow
parse(sun): invalid location
dev_ioctl_send_fail: token = 19
failed to mount /data

/dir/A=RW:/dir/B=RO /dir/my-union fuse.unionfs-fuse allow_other,cow,use_ino  0   0