Don't know anything about minis, I have drafts so can't help you with the length of anything.
Basic elementary set up: with my team they are hooked to a pole (9ft pole that is between the two horses) the pole has a neck yoke which is about 3 ft that each horses collars are hooked to each end. This and the britchen is the braking system to stop the cart and to keep the cart from running over the horses.
I'm not sure if you can do that with a breast collar harness. I'm no expert, but I don't know how you would get your braking. With a single you have the shafts to hook the harness to for braking.