Well, today was ENcouraging finally rather than DIScouraging! I took one of the approaches suggested on horseforum.com of just hanging out with Flash with Snickers loose in the round pen, and pretty much ignored him until he wanted to come to me. I chose to go with the round pen so he had enough room to have his own space, while at the same time not letting him get far away enough from me and Flash to forget we were there. I was brushing Flash and giving him treats, and I initially gave Snickers a treat so he knew I had them. He was decently interested and followed me once he noticed Flash following me, then figured out that if he followed me, whether or not Flash followed me, he'd get a treat. I did approach him a few times, too, when he seemed to lose interest, just to get him used to me walking up, and gave him treats and brushed him as a reward for letting me approach him.
After a while, I picked up the halter. In the week or two before our roping incident, Snickers would let me approach him, but keep me in front of him and then move away when I moved to his neck (I dealt with this by holding the food in such a way that he had to stretch his neck across in front of me, then moved in to wrap the rope around his neck, though that wasn't the greatest approach from a training perspective - obviously). As soon as I picked up the halter, he did the same thing today, not letting me get to his neck and moving away from me when he realized what I was trying to do. When this happened, I turned and walked away from him, as someone suggested on here, and went back to petting, brushing and treating Flash. Eventually, I was able to walk directly up to his side when he was standing against the fence, and he responded by looking at me and I gave him a treat as I rubbed the halter and lead rope on his neck, then brushed him for a while and gave him a few more treats, then went back to Flash (who was quite an [adorable] pest during this whole exercise, not wanting to be ignored since he knew how to earn the treats! He also figured out that they were in the bucket, and that if he knocked the bucket over he could reach them... But I managed to keep his attention enough not to let it detract from my lesson with Snickers). Eventually, he had no problem with me approaching and rubbing the halter and lead rope on him, then putting the rope around his neck, and finally haltering him, brushing him, then releasing him and starting over. He didn't really want to follow me much, though - at least not until I started using carrots instead of biscuits. Then he was VERY interested - I think I found his favorite treat! He would follow me, and then let me move around to his right side - the halter was in my right hand, so with me directly in front of him, I had to reach around with my empty left hand to rub his neck. If I move to his left side, with the halter in the hand reaching toward his neck, he moves to keep me in front of him, but he was fine with me moving to his right side, putting the rope around his neck, then moving back around to his left side to put the halter on. Awkward, yes, but successful! We can work out the details later.
After a few really good "catches", I turned them both out in the full size arena to give them a break, especially since Snickers needs turn out time from his stall. I went and did my own thing for 20-30 minutes, then went back to get them. Flash came as soon as he saw me, and Snickers followed behind. He seemed a little interested, but was still stand-off ish since I had the halter in my hand. He didn't wander too far away, though, just walking up and down the fence near where we were and occasionally looking at us. I just pet and treated Flash, attempted to approach Snickers when he stopped moving, and walked away when he walked away from me. Soon enough, I was able to walk up to Snickers, gave him a treat, and walked away. It only took a few tries before he followed me back to Flash, to which I walked up to him, haltered him, treated him, pet him, let him go, and then walked away. He followed me both times that I walked away, which I think was helped by Flash following me as well, but he also seemed to be doing it also because he wanted to follow me. The third time he followed me, I caught him, gave him a couple of extra treats, caught Flash, and called it a day.
Finally, rather than chasing Snickers away, he's letting me come up to him, catch him, and even following me sometimes - especially when I have carrots (though I ran out in the round pen, so the time he came in the arena was without them!) His overall demeanor seems timid and stand-off ish, but seems like he would like to participate if it would be a positive experience for him. I think we'll do this again tomorrow, and hopefully we just keep getting better.