@vansoffthewall First, don’t walk them on a collar. They can develop collapsed trachea. The collar is for carrying ID tags and other information(like medical needs/vet they use/phone numbers/etc). You want a harness. Probably when getting started you want a front pull harness but they will be growing out of them so you will be able to try different ones even if you don’t want too. Lol. You want to introduce the harness the same way as the collar, just slowly and a normal object that’s on the ground and there. If freezing still happens with the leash on the harness, then don’t clip it. Do it the same way as everything else. Slowly. Just another normal object that don’t do anything. Also, slowly doesn’t mean in slow motion like you are stalking a wild animal. Slowly means do a little bit(3-5 minutes), then stop for a while(15+ minutes), and repeat. This can take a few days(normally I would say few weeks but then I remembered they were collies). Don’t rush it.
And now for the bad news. You have two puppies. Did you research littermate syndrome? A reputable breeder will often refused or at least confirm you know fully what you are getting into before you put any money towards it. Good shelters will do this too but I’ve seen stories of people being told nothing anything because no one told them. Two puppies isn’t easier. Because they NEED to be kept separately. They need separate walks, separate kennels, training, play, potty, everything. They of course can play with each other but that’s such a small amount of time in the day. I’m talking like 30 minutes sometimes if they start to show signs of littermate syndrome. Littermate syndrome is attachment to each other, difficulty bonding with humans and other animals, aggression, separation anxiety, and reduced independence in training. Ever seen or read where the red fern grows? The ending is often times the ending for a lot of dogs bonded like this. But the fighting that can also happen(not in the book/movie, in real life dogs I’ve seen) is some of the scariest. You got a slight leg up though having a male and female.
If this is something new you are hearing about you might really want to do some research on it and figure out if this is truly something you can handle because more often then not dogs like this get rehomed or even abandon because the owners can’t deal with the behavioral issues. If this is something you already know all about then ignore me! Im just here incase someone else don’t know then. Lol.