Note Multiple checkout is not possible unless it is enabled by the VSS administrator.
By default, VSS prevents more than one user from checking out the same file at the same time. However, your VSS administrator can allow multiple users to check out a file simultaneously.
When multiple checkouts are allowed, VSS keeps track of all users who have the file checked out at any given time, and compares changes made to the file when each user checks in changes. In most cases, users have changed different lines of code in the file, so VSS simply merges the changes into the master copy of the file. However, if VSS detects that two users have both modified the same line of the file, VSS displays a message alerting the user checking in changes to resolve conflicting line changes before proceeding. When this happens, no checkin occurs. The user receiving the message should edit the local copy of the file in the user's working folder to resolve the conflicts.