Thank you for posting on our Community. Currently the 2-way Sync for Cross-Doc is not supported, but we have taken down your request and shared it on our Feature Request list along with some others. We have also assigned your email to the request so that should the status change you will be notified.
You can update something with buttons currently, but I don’t believe that will help you with your particular request at this time. While I don’t have a firm ETA on this feature I know the team is working on it. I’m sorry I cannot provide a more concrete deliverable, but I want to thank you for providing your vote, and feedback which will certainly make Coda better as we continue to roll things out.