Just tried to use "-d" and learned that it's a GNUism which isn't available under MacOS, so it's not a portable solution. And neither was it available under BSD 4.3 when I learned about xargs the first time.

Sure, but my example was just that and I actually use /identical$/ as the pattern. Sorry for the typo.

And I use this "historic" one liner only when I know about the contents of both directories. As soon as I need a "safer" solution I use a Perl script and pattern matching, as I said.