Official comment
Truly simultaneous editing of a document by multiple people is not a scenario for which file sync and share products are intended, including Syncplicity. We want Syncplicity to be a useful and productive tool for you, but we caution you not to expect multiple concurrent editing to be solved by Syncplicity. You should investigate a content management tool for that scenario, and be skeptical of file sync and share solution that suggests it handles this scenario.
Syncplicity will create versions of the original document when it detects multiple updates of the same version - which is what happens when multiple edits are made at the same time. These will be identified by changing the file name to add the user who made the change and a sequence number. These new files with the version information in the name will be synced to all the users who have the file shared. Merging the changes into a single document and deleting the versions is a manual process.