Quoted:
i have french doors on the back of my house and i ptu a lock at the top of the doorframe. not a deadbolt but the type you slide the bar into. you cant see it from outside and unless the dude breaks out the entire window frame and each section of glass to stick his head indide he wont be able to figure out why the door wont open. If he goes through with breaking out the entire door though there isn't much point in a lock is there?
by the top of the door i don't mean between the two doors either. I mean where the top of the frame meets the top of the door. criminal might feel around between the doors but i doubt most will want to keep breaking glass all the way until above the center of the door in hopes of finding a lock.
This.
French doors are notoriously hard to secure.
Your best bet is to bolt each door, both top and bottom, into the head of the frame and the floor where they meet in the center.
That in addition to a standard entry lockset between them.
Good luck if it's your main entry, you'll need at least three keyed bolts.