Is it possible to do a bidirectional pull request sync with copybara?

Hi, I am trying to use copybara for the below scenario:

There are two github repos.
- external (public github repo)
- internal (private github repo)

When we open a pull request in external repo, a pull request is automatically created in internal repo with the allowed contents of the external PR. I am achieving this using copybara's `CHANGE_REQUEST` mode.

Whenever a new commit is added to the external repo PR, the internal repo PR automatically gets updated (by copybara force pushing a squashed commit of all the commits in external PR). so external -> internal commits get synced properly.

Now, I wonder if we add a new commit to the internal PR, is it possible to sync that change back to the external PR?

Has anyone succeeded in doing a pull request to pull request sync and more specifically syncing back the commits from the auto-generated pull request of copybara back to the original pull request?