Sync of exist folders

hello, we had in the past an working own cloud installation with two clients and files about ~180 gb. for some reasons the first installation broked. after 3 month the new own cloud installation was set up. now its time to sync the two clients of the first installation again. in the past 3 month some folders was changed on the one client and some on the second. some folders maybe on both.
if i start a new folder-synchronization on both clients with already exists folders - do the oc-client know to sync the files or its better to do it manually?