Braid someone a little bit. Unless the horse you braid has an absolutely ridiculous habit of rubbing and wreaking havoc in the pasture, it will stay in. I would suggest the forelock because it would be more obvious in the distance, I think. And like Solon said, braid in a really colorful ribbon.
Another suggestion would be to wrap someone's tail in vet wrap. But, then you'd have to buy vet wrap, and it wouldn't stay in as well or as long. =]
Oh, and I wouldn't feel bad about not being able to tell them apart. While we can see the differences in a picture that close, they would both be white horse-shaped blobs from a distance. It's interesting that you got such look-alikes. =]
I guess it wouldn't hurt any to try a braid, though; I'll put one in when I go riding tommorow and see how long it lasts.
And they're mother/daughter (Tanner is the mother) -- they look exactly like Tanner's dam, too. XD The last owners kept different colored halters on all three of them, but their pastures were really open, compared to my heavily wooded ones.
Magic is white-Gray and Tanner is Fleabitten gray, though those fleabitten spots will fade as he gets older.
Magic has bigger ears and Tanner has a smaller muzzle. But you're right, they are quite close in look. Is there a reason you need to be able to tell them apart when you're far off? As long as you can tell which is which close up, it shouldn't really matter.
It's not like I plan to spot them from across the pasture, I just like to be able to glance at a horse and know which one it is quickly, without needing to get close enough to inspect her back legs for scars or whatever. For instance, Bandit and Dante are pretty similar in looks, too, but Dante is a shade lighter and Bandit has more muscle, so I can tell them apart when they're out in the pasture, but I still have to get within a few yards to tell them apart.
For Tanner and Magic, when I go out to catch them, they don't always stand still long enough to be caught easily (it's being worked on), and I've done a lot of catching Magic when I meant to get Tanner, or vice versa, and then I just have to catch the other one.
They are very pretty I say dye one purple, hehe jk. What about a tailbag for one or both of them in different colors? That way the braid will stay in too.
And I sorta considered finding a semi-permanent horse hair dye and dying a star on one or something. >.> I'm planning on running them in barrels, and I was going to paint some kinda symbol on their flanks anyway (because the judges at our local show got confused the year I showed Dante in weanling halter and Bandit in yearling halter, so I figure I'll mark them so the judges can tell they're not the same horse. )
With the combination of Dante's chewing on everything and the forest, I'm not sure a tailbag would last very long.