Feed bags may work, but if the aggressive horse realizes there is grain in the pony's bag, he is likely to try to rip it off.
I had the same situation with two of my horses who were kept out 24/7 with a run in shelter and one would finish her feed before the other and then go after his. I fed them hay on mats at opposite sides of their shelter, and put their hard feed in rubber bowls on top of the mats. Then I installed tie rings to the wall and tied them so it allowed enough length for the horses to reach the floor to eat but not to get tangled in. The mare would always finish first, so by tying her to eat, when she finished she occupied herself with the hay until the gelding finished his hard feed. I actually never needed to tie the gelding since he stayed with his own feed until it was gone. So, I'd first throw the hay. Then the mare would get her halter on and be tied, and be fed her hard feed before the gelding was fed his. While they ate, I filled the time by mucking the shelter and area around it, filling the trough, etc., and it worked out well. When the gelding finished his hard feed, I removed the mare's halter and they got along fine.