How to enable fall-through directories in Linux, so if /path0/file doesn't exist, path1/file is used

Question

I'm seeking way to have linux fall-through on directories so that with this structure:

/dir0/a.txt: "A"

/dir0/b.txt: "B"

/dir1/c.txt: "C"

/dir1/b.txt: "BBB"

dir1 falls back to dir0, leading to:

cat dir1/a.txt: "A"

cat dir1/b.txt: "BBB"

cat dir1/c.txt: "C"

Answer 1

http://www.filesystems.org/project-unionfs.html