WRT the physical aggression, can you try using words that don't bring hitting/kicking to the forefront of his mind? Instead of "We don't hit or kick!" say, "X, you need to use your gentle hands and feet." and show him what gentle hands and feet are, by touching his face or arm, etc. softly.
Sometimes kid brains get caught on a negative word and they literally have to think of that word - the old "Don't think about pink elephants!" thing, KWIM?
Also, although I have used (and do use) harnesses, I think a single stroller sounds more effective for your charge. "X, we are going to walk. If you can't walk or get too tired to walk, I will put you in the stroller until you are ready to try and walk again." Then when he droops to the ground, say, "Oh, you are too tired to walk? Well, let me help you climb in the stroller!" You can teach both kids to hold the stroller instead of your hand.
Basically, you want to give positive praise when he does well, and ignore/give minimal attention when he doesn't. Also, if his twin is walking nicely, praise her for that, or for other behaviors he needs to mimic.