First of all, congrats to opening your mind to other training opportunities! You are on the right track and doing well thus far, and of course there will be a lot of learning curves as you find your way through this new method. [

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A few observations:
On your walk you referred to "they". Were you walking both dogs at once and attempting to click and treat them on the walk? If so, I'm not surprised it didn't work out. When doing clicker work, at least at first, work only one dog at a time. Working both dogs at a time will set them up to get confused and perhaps frustrated.
When discussing the idea of "re-training", you have to take a step outside of the box and look at it from another angle. You say your dogs already know how to LLW - right, they know how to do it,
when on a prong collar. They have learned that when the prong collar is on, they have to walk nicely or else they will get a pinch (ouchie!) when they forge ahead. So they walk beside you. However dogs are clever beasts, and will learn the difference between when a prong is on and when a flat collar is on, and they will behave very differently as a result. Your dogs do not know "walk on a loose leash regardless of collar", they know "don't pull while wearing the prong collar because it'll hurt if it do!". So basically they really don't "know" loose leash walking at all, which is why you're seeing what you're seeing happen on a flat collar!
So it's not a matter of re-training at all, but TRAINING your dogs that walking beside you brings GOOD things to them. [

] It brings walks to occur at all, it brings treats, it brings opportunities to sniff the environment (if the dog is pulling - walk stops and dog has no fun...dog will learn to walk to get what it wants), to meet other people, etc. So to train it, do it just as though the dog didn't know anything to begin with, as if the dog had never walked on a leash before, and forget how the prong collar made them behave.
On the same note, I realize you can't train all the time, and when dogs need to get out, they need to get out. Now, you could use the prong collar in between training sessions for management (management is a big part of the clicker ideology - preventing unwanted behaviours from happening between training so your dog can't build a reinforcement history). Or, since you are delving into the world of a new way of thinking, perhaps you might look into a front-clip harness for your dog. It's a non-painful way to control pulling in dogs, and it can be used as management when you can't train and need to get from point A to point B. Or you might look into a GL, but I'm not sure how well they fit bully breeds (I've never fit one to a bully breed). But it's a consideration you might look into.
And really listen to Mudpuppy's advice on training sessions. While you can stick a prong collar on a dog and use it right away for 2 hours at a time, the clicker is not that way. Because it requires dogs to actively think, to engage those little brains of theirs, it can be very exhausting for them (in some cases more exhausting than that 2 hour walk!), so especially for dogs new to clicker training, stick with short sessions. Even with my clicker-savvy pups I don't work more than 7-8 minutes at a time, and for puppies it's much less time than that. So in effect, do a LOT of training during the day if you wish, but break it up into little sessions. I think you'll find it easier to train that way.
And if you are one of those people who live in the real world, and your dogs need their walks, for now separate walks and training time. Train for the walking behaviour you eventually want to see, but don't totally deny them their walk or make their walk INTO a training session if you go for a long walk, because dogs do still need their exercise and if you're trying to get a walk in while training it can end up being frustrated. So walk your dogs during walking time (using management for now), and train for what you EVENTUALLY want for walking time, during training time. And eventually you can put the two together and get them to become one. At the same time during walking time, bring your clicker and treats if you like and reward when the dog DECIDES to walk next you. It can be an informal training session and the dog will learn a lot from it as well, and it's usually more relaxing to do that way than a formal training session.
Kim