I wouldn't...

...put horses to a cart like this.

...let alone mules.

First of all, I know you don't step them over the shaft, but back them in.

And the ass here is the guy in the purplish shirt. You can't pull an equine back by the breeching strap. He keeps messing around behind the mule and then he's dropping the chain, which them allows the right mule to swing away from its partner...that's a butt chain and needs to be hooked up quickly. Finally, the mules have enough and vacate the premises, followed by extremely loud shouts of whoah that aren't going to do any good.