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 (including sources and destinations) by clicking on Add more bases and then on the + button underneath the list.
One-way sync / One-way move
Multiple sources are 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.
Note: multiple sources are meant to virtually merge files spread between different directories or even volumes. If you want to keep the source directories separated on the destination, it may be a sign that you should use the parent directory as the source, exclude it and include only the desired subdirectories (see Filters).
Two-way sync
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. You can read more in Sync types.