There are two probems with sharing the library folder (regardless of method):
- Do not run the sync program while calibre is running, or you will inevitably suffer conflicts when obsolete files are preserved, or changes are reverted, since the two cannot agree on what files were there.
- Always sync fully before and after using calibre. Do not ever open calibre with a non-updated library, or you will end up with two slightly different libraries, or conflicted copies or the library. Alternatively, if using filesync programs instead of cloud storage, you can use a one-way sync from the canonical library to backup/mirrored libraries (so it's all right if they are out of date, as they won't "update" the true library.)
For Bittorrent Sync, if you can set it so only one computer can update the torrent you should be golden.