Synde,

http://www.addalock.com/burglabar.html

Many hardware stores offer a product that clamps on the metal track and
can be used to lock the door partially open for pets. These clamps are
convenient but are not very secure.
If you close your door and carefully drill through both frames where they
overlap in the middle you can slide a bolt or nail in the drilled hole to
lock the doors.

Dave M.