They are temporarily hidden under that second mounted filesystem. As soon as that filesystem is unmounted, they will reappear.
a way to access them even while that second filesystem is still mounted. Take a look at the manpage for the mount
command and scroll down to the section, "The bind mounts." If, while both filesystems are mounted, you do
mount --bind /someDevice /mnt/tmp
you will then find that /mnt/tmp/SomeDirectory contains the contents that were there before the second mount.