Whilst a surface mount espagnolette is probably better than simple window catches (2 locking points, so better resists jemmying) - a far better solution are espagnolettes built into the frame. This is quiet a bit more work for the joiner, but is the solution you see on all the upmarket modern timber windows.
Built in will allow you to fit and aftermarket europrofile quarter-turn handle, with key locking if you wish - the mechanism is entirely concealed in the frame. If you need high security consider getting the glazing sealed units with toughened or laminated glass (approx + 40% for toughened, but glass is a small part of the overall cost). Specifying smaller glass panes helps too.
I've recently made a set of oak, gothic arched chapel doors and bought multipoint locks from these people
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- I found them extremely helpful - even when I had to change some parts. I used lockmaster branded stuff this time (exactly the right design for my doors) - but have lived in a flat with windows using maco hardware, which was outstanding.
Do you mean full height double french doors as windows? If so the Lockmaster French door locks may be exactly the right solution for you (they require a false mullion be built into the slave door, the payoff being lots of locking points)