PathCommonPrefix

[DllImport("shlwapi.dll", CharSet = CharSet.Unicode)]
public static extern int PathCommonPrefix(
   [MarshalAs(UnmanagedType.LPWStr)] string pszFile1,
   [MarshalAs(UnmanagedType.LPWStr)] string pszFile2,
   StringBuilder pszPath
);

Microsoft documentation: Link