I'm not sure I understand why you describe Dropbox as one-way. If you
share a folder with someone, it appears in both of your Dropboxes and
any changes people make to the contents is shared with everyone. It
worked quite well with a 3-party project I was working on recently.
I'm sorry if I'm missing something here, but wouldn't Google Docs or
Zoho fix this problem? I use Docs to collaboratively write articles
and proposals with my students and it works like a charm.
Pardon me if i am missing something, but Google docs (docs.google.com) seems
to do this very well.