You've got the right idea, LTK. But you need to put your pants on one dog at a time, so to speak.
Teaching both of them at once will be nearly impossible. It would be best to teach them each to "heel" separately, then when they've each got it down you can try walking them together.
The method you're using--stopping when they pull, then allowing them to continue when they let up--is one of my most frequently used methods. It really works well when it's done correctly, because the dog receives an automatic reward when he stops pulling: he gets to keep exploring. Then, if he pulls, oh! he has to stop again, denying him his reward. But when you've got two dogs at once, one may stop pulling and have to wait around for the other to stop pulling before he can get his reward. So really, it's more rewarding for them both to pull because they will get their exploring time while dragging you down the road. New concepts always take some time and lots of trial-and-error to sink in. Then, it "clicks":
Oh, if I keep this thing loose around my neck, I get to play! But trying to teach them both at the same time will make it just about impossible for them to get that "click". They won't understand what's making you stop and start, because they're each doing different things.
Remember to give the command "heel" (or whatever you use)
as the dog is performing the action you want him to associate with the command, not before or after he does it. Only give the command if the leash is loose, so your dogs can learn what it means. Then give a treat or a pet on the head.
This method does take some time at first, but most dogs do really well within 4 or 5 walks, depending on how far you go and how well you stick to the method. So in 2 or 3 weeks, give or take, when they've really got "heel" down pat, you can try walking them together. I'd bring a friend with me, so that we each could walk a dog. You want to take training "baby-steps"; only add one new challenge at a time. (First you get a loose lead, then you get two dogs on the same walk, then you get two dogs on the same walk with just one person.) But when you do, expect some behavioral backsliding. They'll probably act like they've completely forgotten everything you taught them. They haven't, but give them a little break anyway. Just keep rewarding that "heel", and they'll get it.
