When both doors are operable, different hardware (more expensive) is needed for secure locking. When a single door is operable, less expensive hardware is needed.
One common approach for locking when both leafs are operable is a cremone bolt, on one leaf, which secured the door to the floor and header above. The remaining leaf is dead bolted to the leaf with the cremone bolt.
Alternatively, some French Door sets can be ordered with an intergral fastener on one leaf which locks into the sill and head.
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