Well, I know very little about cart training dogs, but quite a bit about training dogs in general - it's what I do for a living, after all. From your description, this would be my advice - put the cart training COMPLETELY on hold, and work on your obedience. It's pointless to try to work on cart training, agility, herding, or any other dog sport without first having an excellent obedience foundation. It's like trying to break a horse to ride without ever giving it any groundwork first - you might achieve a little, but in the end the work you've done will unravel. You are quite literally putting the cart before the dog!
Get them where they will respond to your commands, come when called, stay, and heel at the very least, all off leash before you even consider adding in the extra training (and not to mention the additional risk!) of hitching them to a cart.
I missed if you mentioned about their ages, but this has been my experience with working herding dogs. A dog is not mentally ready to handle more advanced training until they have been given time to mature. My pups get behavior tested on the sheep at 6 months of age, after that they play with stuffed sheep, socks full of wool, and they watch herding from the other side of the fence - they don't get put on sheep themselves until they are (usually) nearer 2 years old. They just don't have the very strong obedience foundation they need before that point.
Duke is four years old, I got him as a play buddy for Hazel this year. His owners contact me (I had an ad on Craigslist) as they went from country to Suburbia and felt it's just be better for him to live where he can run and play all day.
He's well trained, sits, lays down, heels, comes when called, goes into his kennel, and stays on command, both on and off the leash. Although he is a bit of a lemming (if Hazel does something, Duke blindly follows). I can take Duke out off leash around the neighborhood and he'll stay with me at all times, he's just a bit of an air head. If he's really excited (most of the time) and I tell him to sit, he'll instantly sit. Then if I go to pat his head he'll drop to the floor like a rock all 'You want me to lay down?! Okay! Okay! Okay!'
If Hazel told him to jump off the next cliff, he would sniff one out and merrily leap off into oblivion. I love him, but sometimes he's just...dorkish.
Hazel is year and six months, I got her at a year old completely untrained other than well socialized and well loved. Her owners rehomed her as they had both broken hips and countless other bones in a nasty wreck. Untrained dog who likes to climb into laps (fixed her of that fast, now she only does it if asked) plus broken bones just didn't clash well and they had no one capable of taking proper care of her.
Her training needs to be constantly changed and kept working and interesting...otherwise she just gets bored and tunes you out. One day I could be working on sit and stay, the next it's sit, heel, and pop over this jump. If I do sit and stay two days in a row, I'm setting myself up for frustration and a bored, useless dog.
She sits, lays down, stays (in and out of sight), comes when called, and heels off leash...when theres not something else around, like another dog. If theres no distractions, then she's a very obedient, intelligent girl.
I've established the basic 'you listen to me when I tell you something' and I'm currently working with her on 'you listen to me when I tell you something, no matter whats happening'. Her favorite thing is rabbits, I currently have a retired brood doe acting as my 'wild rabbit'. We're still doing on-leash stuff, she has to sit, lay down, stay, heel, and walk quietly no matter what the bunny happens to be doing. She's progressing at a good pace, but still needs work. She has half her attention on me and half on the rabbit, I want it completely on me and none on the rabbit :)