How can I merge two directories using opendiff?

This should be simple, right? I want to compare two directories, foo and bar. I want to cherrypick changes in foo and place them in bar. If I run

opendiff foo bar -merge bar

to specify bar as the merge directory, nothing happens when I "save merge" on my individual files. If I don't specify a merge and select bar as my merge directory from the save dialog, it gives me an error.

Surely I'm missing something?

From stackoverflow

• Here's documentation on opendiff:

  http://www.manpagez.com/man/1/opendiff/