Entry through a patio door is a common means of entry and therefore, unless the doors are fitted with a multi-locking system, it is best to fit extra locks. These are fitted on the bottom fixed frame pushing through to the sliding frame in the centre and on the side frame at the opening point no lower than a third of the way from the top of the door. Most such locks are push to lock and key to open and are therefore easy to use. Make sure there is enough frame to fit them on if drilling is required. If in doubt, consult the installer or manufacturer.
A dual screw can be fitted between both frames but it is more awkward to use, so there can be a tendency to leave it unlocked, thus defeating the object. An alternative would be to fit long-throw bolts top and bottom of the opening door.
Some patio doors can be lifted off their track. If you can lift it more than 1/4″ simply screw wood blocks of a suitable depth into the channel above the opening door to prevent this. Alternatively, anti-lift devices are available from locksmiths.