How can I create an union merging / and another directory, on / itself?

Say, we initially have / with the following contents: /bin/foo /usr/bin/bar /usr/lib/libfoo.so We also have another directory, say /stuff, with the contents: usr/lib/mystuff/mystuff.so etc/foo/qux/buzz bin/stuff Is there a way to create a union between / and /stuff, so that I get: /bin/foo /bin/stuff /usr/bin/bar /usr/lib/libfoo.so /usr/lib/mystuff/mystuff.so /etc/foo/qux/buzz (with all of the content from /stuff there as well, just not shown there.) This union should be of a **temporary nature** (just like filesystem mounts are), otherwise I could have used cp, mv or rsync as well. How should I go about doing this? _One would expect that an answer to this would be easy to find; and indeed, there are many online resources that mention creating unions between two directories and mount them elsewhere, but none really cover this particular case of mounting a directory "over" /._