I mean, if they are going to get shot at directly like that, I don’t see how the second barrier is going to increase the risk. Best escape still would be an acceleration in the direction they were already headed.
They could of course have bullet-proof vehicles, but double-wall barriers are pretty popular border designs for a reason.
(Alternatively, I wouldn’t mind filling the “moat” bottom with concertina wire and having them drive within the second barrier—but they would presumably need to have access to that moat space.
I see what you mean, but it’s still my opinion that being confined between the barriers would not be the best option.
As for what to put between the barriers, how about some buried fougasse-type devices to spray intruders with something like skunk scent ,possibly mixed with dye? They could be deactivated if the patrollers had to enter the space between the barriers (though you might want to send the new guy in first, in case they were still active).