Two individual changes to sites on two PCs could cause sync conflicts.
The chance of a data sync conflict is extremely low.
Multiple machines would need to save/sync at the exact same time.
Before reading/writing any changes to the data files FlashFXP checks to determine if the data file on disk has been modified since the last access, if the file was modified then FlashFXP will reload the data first.
btsync (now known as Resilio Sync; but I use the older btsync) is by far the most aggressive network file sync app I have ever tested/used with FlashFXP and to this day I still test sync data across 4 machines. 1 always on, 1 on most of the time, 1 rarely on, and the last on and connected over wireless. Plus I use application password protection and that requires the files to be 100% accurate all the time. This leaves zero room for error.