Hydra

# make dir1, one some files and folders in it

mkdir dir1
mkdir dir1/dirx
mkdir dir1/diry
touch dir1/filea
touch dir1/fileb
touch dir1/dirx/filep
touch dir1/diry/fileq

# make dir1, one some files and folders in it that also appear in dir1 and some that don't

mkdir dir2
touch dir2/filea
touch dir2/filec
mkdir dir2/dirw
mkdir dir2/diry
touch dir2/diry/fileq
touch dir2/diry/filer

# combine the folders

mv -f dir1/* dir2                         

# gives: "mv: cannot overwrite directory `dir2/diry'"