I use a surface mounted vertical deadbolt, aka "jimmy-proof deadbolt" mounted on the inside of the primary door and the striker on the secondary door which is also secured with slide bolts top and bottom.
If the slide bolts fail the knuckle action of the deadbolt would prevent the doors from opening more than and inch or so.
One the outside the only visible part is the keyway cylinder. On the inside the bolt is operated with a turn knob. I added simple pull handles inside and out to overcome the friction of the tight fit and weather-strip.