When all different users (there may be more than 2) are working on separate datasets (there is no data entered or edited by several users), I have used following setup:

1. Every user has own workbook. All workbooks are on some network resource(s), and the location(s) of workbooks are kept unchanged;
2. Every user has his own dataset for entering data and editing them;
3. Whenever some user needs data entered by other user, this info is read from other user'd workbook using ODBC query (The query from other sources>Excel files). Result tables of such queries are used as sources to get needed data through formulas or data validation lists;
4. Queries are set to be refreshed on opening, and the data read from other workbooks are from time the workbooks were saved last time. User can also force the refreshing of all queries at will (Data>Refresh All). As queries are read-only, they don't lock other workbooks - every user can work with own workbook freely;
5. To be this setup effective, all users must collaborate saving their workbooks whenever some essential changes are made.