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Well, lots of times when you take them right away from home, some where completely different where their whinnies aren't answered, they're ok. Especially if there are other stranger horses around. One thing you could do, if this is available to you, is put them in seperate paddocks, where they can see and hear each other, but not touch. They won't like it at first, but it wont be as dreastic as taking them completely out of sight. You should get them used to talking one of them away (probably cookie since thats the one you're going to show), throw the other a flake of hay, and put the one you've taken out in a place where they can eat too. If you do this regularly enough, they get used to it, and the herd boundness gets better.

Okay, I'm going to go into a bit of behaviorist musings here, so bare with me. I don't know how helpful it will be, but... well, sometimes my musings wind up somewhere somewhat productive.

Seeing as Cookie was the one who got nervous, and is also the "low man" in the herd, I would say that the problem is less specifically Jasmine being gone, but Cookie's insecurity of being the "lone horse". The alpha horse is actually a position of considerable responsibility. They have to be the ones to watch out for danger, and let everyone else know what to do, where to go, etc. Some horses naturally step into this place without question from any other horse in the field (my thoroughbred was like this), others need to constantly reassert this dominance (Griffin was like this) and other horses would be much happier just to let someone else tell them what to do (Danny was this one), and there is everything in between. When it comes down to it though, in the wild, a lone horse is a dead horse. The horse that is the "alpha" of the herd is more confident being alone because they're used to having to watch out, to control their surroundings. The more submissive horse will panic because suddenly, their protector is gone. It might seem like Jasmine is always chasing and biting at her, but Cookie probably depends on Jasmine for a lot. My thoroughbred and the quarter horse he lived with displayed the same behavior. Mack didn't care one way or the other if Woody was around or not. I'd take him trail riding, take him out to work, whatever. Didn't phase him. Woody, on the other hand, would go into a completely spastic attack if Mack left his sight. Mack was the uber-dominant one, Woody was submissive. I think your key to solving your problems here is going to lie in convincing Cookie that you are also part of her "herd", and that when you and Cookie are together, you two are the herd, and you are the alpha. She'll need to be confident enough in you to depend on you for her safety. From your first post, I'm guessing that you're somewhat into the natural horsemanship movement, which is a great way to build your horse's confidence in you. I'm not the biggest Parelli fan in the world because I don't like the over-commercialization of the "savvy system", but I will acknowledge that those people are marketing GENIUSES (we're studying them in my equine business class), and I'm taking a natural horsemanship course at the moment and the theories are sound. Ground work is very helpful for helping to build your horse's confidence in you. Where do you normally work with her? Is it within sight of the pasture where Jasmine lives? How confident is she normally with the seven games (assuming that this is the program that you're working her through, if not, ignore me. It's been a strange day)? Try working with her in an area that is near where Jasmine is, so that she can still see and hear her comfortably. Work your way up to taking Jasmine away completely by moving her farther away, and show Cookie that she can still depend on you to fulfill the leadership role. As she gets nervous, keep her busy. Change things up and keep her thinking. Add new challenges to exercises you've had mastered for a while. Keep her mind engaged and focused on you. You might be tempted to focus on easy simple things, since she's already stressed, but the more she thinks about Jasmine not being there, the more worked up she'll get. So get her attention, and then keep it by making yourself and what you're doing as interesting as possible. Not scary, not intimidating. Just interesting.

I'm not sure entirely how helpful that was, I kind of circled around a few topics a few time and I'm not sure if I ever alighted on them, but hopefully it was somewhat useful.

hm, apparently I forgot how to read then. Sorry about that!

Little Update: I took cookie out of the paddock and went down to the round pen, she was fine, not nervous or anything, Jasmien was still back in the main paddock, and she was good too, i didn't hear her and neither of them were talking. Me and Cookie did a nice join-up sesson too. ^^ The factor of cookie only gets nervous is when both of them are out at the same time might contribute.