A rolling solid gate I have needs to be opened from either side with a key to prevent someone from jumping the fence and unlatching a typical latch. At present I have a flat steel bar inserted through the side of the gate frame and on through the fixed post that the gate closes to. There is a padlock on each side of the post/frame on the bar and I can use one from the alley and the other from inside the gate. It closes up to a wall so I can't go around. It's one or the other.
I installed hundreds of sliding patio doors, some with an outside keyed entry. None of them had an inside keyed lock unless I added one at the top or bottom. I've never seen a double keyed lock for a sliding door or gate all-in-one. And I can't find one.
The Pelican brain trust is tasked with finding a solution that would be outside or barn door friendly, better than a wrapped chain and in no way decorative. This needs to be a 3 feet or so, not at the bottom and there is no head jamb to vertically latch to. So that rules out Euro multi point latching mechanisms aside from the fact that they are very expensive.
I've replaced these before at $300+ea. plus tax and shipping. Call it $400 all in. Not gonna happen on an alley gate. I can't use a mortise lock anyway, it has to be surface mount on the inside with a lockable function from the solid blank outside.