In the General tab of the sync settings, where you can define the source and destination of a one-way sync or one-way move, or the two bases of a two-way sync, you can define additional bases by clicking on Add more bases and then on the + button underneath the list.
You may also read more about sync types.
One-way sync / One-way move
Multiple sources will be virtually merged and mirrored to all the destinations.
Files with the same name on different sources but with different modification dates or sizes are considered a conflict, causing an error to be logged. If a file has identical copies on two or more sources and only one of them is modified, to avoid a conflict all the old versions on the other sources would also need to be manually replaced.
A file on the destinations is deleted only if it is not present anymore on any of the sources.
A two-way sync always behaves the same, regardless of how many bases it contains. In particular, between syncs, a file may be modified only on one base, otherwise it won’t be possible to determine which change should be mirrored to the destinations, causing a conflict error to be logged.