Right, I understand the basic purpose, and that would exmplain why the T&M manuevered to have the new Macclesfield install a stop gate, but doesn't explain the reason for then building their own stop backed right up to it, except for obvious redundancy in case something happened to the Macclesfield stop gate. And, maybe, the explanation it's just that simple (redundancy).
But, as has been discussed, this was a very unusual setup. We're talking two chambers with four sets of doors in which both locks could act as lift locks; in fact, it seems arguable that that was their primary function. Typically, these canal junctions would only have two doors, correct? I think the other thing is that when we're talking about a "lock" we're usually talking a pound enclosed by two sets of doors to provide a rise/fall, whereas a "stop lock" is just a gate, right?