The fixed door on many french doors is held closed by a pair of barrel bolts that sink into the top jamb and threshold of the doorway.
Securing wooden french doors.
However a deadbolt that locks both doors together and single sided deadbolts in the top and bottom of both doors are a good combination for securing both doors.
Install a commercial grade double sided deadbolt into the active side of the door or alternatively install a double side high security deadbolt into the active side of the french door.
Out of all the locking options to secure a french door a single sided deadbolt is the best option.
French doors are difficult to secure for several reasons involving the following weak spots.
Traditional wood french doors have many small panes of glass.
This alone will make it a lot more difficult for a burglar to force the doors open.
Three point locking mechanisms are best for french doors.
When french doors open outward the hinges are exposed on the outside.
Because french doors have no center frame or support they are very weak in the center.
Even if these are tight enough to prevent the fixed door from.
Add one side half deadbolts to the top and bottom of each door.
For doors that come as a pair consider locking securing one of them permanently.
For doors with a bolt that slides to lock along the top and bottom of the door you can permanently lock the door by drilling a hole into.