Sharing Dropbox files between Ubuntu and Windows
Solution 1:
I see only one possible not very handy workaround in that case: to use Dropbox application only on Windows and to share (by mounting NTFS partition) the Dropbox folder with Ubuntu.
Pros: There will be only one folder without duplication.
Cons: Under Ubuntu it will be possible to read and to write to Dropbox folder. But for saving changed files in Dropbox cloud, Windows with Dropbox application must be started to perform needed update.
The similar question on Dropbox forum: Write to a Dropbox folder on NTFS from Linux