All you glider pilots who fly a K21 two-seater, BEWARE!!!!!
I have seen the K21 canopy and the flimsy mechanism that is supposed to prevent this. But it only prevents locking of the forward canopy if the rear canopy is not locked. It does not prevent a subsequent opening of the rear canopy, either on the ground or in the air. Furthermore, the nylon blocks that form the mechanism wear easily, and in one K21 there's evidence of damage where somebody forced the front closed with the rear open, and succeeding.
At the club where I learned to fly gliders it's customary to put pax, who were there for an air experience flight, in the back. Obviously with strict instructions not to open the canopy in flight. It's an accident waiting to happen, but for some reason flying the K21 from the back seat is a privilege reserved for instructors. Normal GPL pilots with a passenger endorsement (a club thing) are supposed to fly from the front seat.
Mary, the rear canopy departing altogether, was that a lucky break, or is there a mechanism, weak link or something else that ensures that the rear canopy departs in this scenario? I did witness a canopy opening during the winch launch of a Junior once, and there the pilot had the wits to pull the emergency hinge release handle so that the canopy departed altogether. But I have not seen anything in the K21 that would allow the front seat pilot to release the rear seat canopy.