A docker container running mergerfs.
docker run \
-v $PWD/one:/disks/one \
-v $PWD/two:/disks/two \
-v $PWD/merged:/merged:shared \
--device /dev/fuse \
--cap-add SYS_ADMIN \
homespirit/mergerfs
The above command starts a mergerfs container which makes the files in $PWD/one and $PWD/two available under $PWD/merged.