How to include part of another git repository using git subtree and merge updates in both directions

How to include part of another git repository using git subtree and merge
updates in both directions

I have two git repositories show below. The first is structured like a
typical python project.
foo_repo/
.git/
setup.py
foo/
__init__.py
some_code.py
tests/
bar/
.git/
I would like to include the foo_repo/foo/ directory in bar/ as a subtree
and I want to be able to merge updates to foo_repo/foo/some_code.py both
from the foo_repo repository to bar and vice versa.
The initial setup isn't too bad. From the foo/ directory I use:
git subtree --prefix=foo/ split -b export
Then I have a new branch in foo_repo with only the contents of the
foo_repo/foo/ directory. To bring this into bar, I just go to the bar/
directory and:
git subtree --prefix=foo/ add ../foo_repo/.git export
Now that I'm all set up, I'd like to do some code development and keep
foo/ up to date in both repos. Pushing from bar I think I have figured
out. From bar/ directory:
touch foo/more_code.py
git add foo/more_code.py
git commit -m "more code"
git subtree --prefix=foo/ push ../foo_repo/.git export
Then from the foo_repo/ directory:
git checkout master
git subtree --prefix=foo/ merge export
Merging the other way is where I'm stuck. From foo_repo/:
git checkout master
touch foo/yet_more_code.py
git add foo/yet_more_code.py
git commit -m "yet more code"
???
Where the ??? is a command that merges the foo/ directory with the export
branch. Then from bar/:
git subtree --prefix=foo/ pull ../foo_repo/.git export
So I'm basically looking for the line that goes in the ??? spot, or a
different workflow that does the same thing. I've tried repeating git
subtree --prefix=foo/ split -b export_foo by that doesn't work.