I don't currently have a buddy sour horse in my barn, but I will make a video demonstrating this technique for you guys.  I have personally been using this technique for years and found it to be effective every time I used it as long as I was committed to success no matter how long it took.

If both horses are saddle broke, take them into an open field or arena.  Park one horse and ride the other off 100 yards or so.  When he wants to go back, let him.  When he gets to the other horse, make him trot or lope circles around the parked pony.  This work should be uncomfortable for him.  Put pressure with your hands and legs as you go around and make sure to make 5 or 6 good trips.  Then leave.  As you leave, take all the pressure off and slow to a walk.  Try to get him to walk off 50 or 60 yards.  He'll likely want to go back, let him.  Repeat this process of pressure near the parked horse and release when he leaves until he doesn't want to come back.  

Hi Dale,  a lot of folks think it is necessary to progress into a shanked bit if they aren't getting what they want in the snaffle.  Personally, I want to stay in the snaffle until my horse understands it 100%.  After that, I might go into a shanked bridle for different reasons.  If I want to show a horse in a particular class that requires a bit, if I want to enhance some portion of my horse's performance and I feel the bit can help, or if I simply want to ride "one handed".  I don't want to progress into a shanked bit as a crutch, though.

Hi Stephen, Watch out for videos with "Pepper".  He's just like what you described.  Let me know if it helps.

"Sacking out" comes from the open range days when a cowboy would rub his horse all over with a burlap sack to desensitize him.  We use flags, saddle blankets, tarps, etc to get the job done today.  Most of the supplies found around the ranch or wagon came in a burlap sack.  Hardware, potatoes, grain, just about anything.  They were also used as saddle blankets or bedroll blankets, too.  Sorry, I forgot to mention this earlier.

Aside from catching one, I rope horses to teach them that there's nothing to be afraid of and to give to the pressure.  I've found the more you rope them, the more relaxed they become.

That sounds just right to me!  What part of the world are you in?

That's miserable weather.

Ok.  I won't have a foal until this spring, but we'll see what we can do then.  Have you watched the videos on ponying?  Thats how I like to do the wilder ones.