The two useful bits from Dropbox et al. are backup and synchronization. The former is useful because the backup is offsite and presumed to be reliable. The latter is useful when there can be multiple sources of changes and simply to automate the process of multiple availability. Neither of these features is present when there are two destinations attached to the same computer: there is no offsite, reliable backup, and there is no multiple availability. When both sets of files are attached to the same host, then there should only be one potential source for changes. With one change source, "synchronization" becomes merely backup, and any conventional backup tool will make sure that the second target always contains the latest copy of the first target. What you are looking for is a backup product.
