To answer the short question, just shared access to the database file itself.
Refer to the following MS articles that explains the concepts succinctly. The process is the same even if the front end is C#.
Office 2007: http://office.microsoft.com/en-us/access-help/ways-to-share-an-access-database-HA010279159.aspx
Office 2010: http://office.microsoft.com/en-001/access-help/ways-to-share-an-access-database-HA010342110.aspx
Office 2013: http://office.microsoft.com/en-001/access-help/ways-to-share-an-access-desktop-database-HA102749577.aspx
You are specifically interested in the database portion of 'splitting a database'