Official comment
To change a file in a shared folder the file must be on a local system or device. Whenever each user saves the file the new version is distributed to everyone with access to the shared folder. If multiple updates are made without syncing, then Syncplicity will create versions of the file that include information about who created each version. These versions are also synced to everyone sharing the folder. If you expect to often be editing the same file simultaneously with other users, it is best not to work on the file in the shared folder to avoid creating multiple versions.
If you get multiple versions you need to manually determine which is the most complete, rename that one, and delete the other versions to get back to a single copy of the data with all the changes.