Mirrored joints is a long-time request around here. Not sure when it will implemented.
I'm not the foremost expert on this, but there are two ways to get your mirrored component to move reflectively with the original. Both require history to be on AFAIK.
1- with the sketch; if you constrain your original bracket sketch to the origin point, when you go back and change how far away from the origin/center line the sketch is, the mirrored component will hop into place after you finish the sketch.
2- with a move/capture position instance; after you've built your component, move it to your desired location. in your tool bar will appear two icons, one for keeping the move, and one for discarding it. Keep the move, then mirror. Now if you go back and update your move instance in the timeline, your mirrored component will hop into place when you save the changes to the move instance.
To my knowledge, components aren't just instantly mirrored to each other where you can drag either one around and the other one does a live reflection.