Confused with sync

Hello, I'm just wondering how does Zotero do the syncs? We are 2 people working with Portable firefox and zotero 1.5b1 with multiple profile for multiple zotero libraries. Usually one of us works with a Zotero library, he syncs it with the zotero server and then the other one press the sync button to get the data from the server, but how does the sync know when the source is local or server? Does it check the date of the items?

Usually each library is edited just by one of us, because we are a bit scared of losing data. But what would happen if we both work with the same library? How would the sync work?

Thank you very much

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