I'm with Laura - tying one behind would be pretty unsafe (although I've been tempted, because the miles add up when you have to exercise more than one horse!).
I have driven one horse with my daughter as a passenger holding the lead on a second horse who walks behind. That works pretty well as long as both horses are pretty well trained on the whoa, and back. The person with the lead really has to pay attention to the ponied horse, though, otherwise they could catch a leg in a wheel, or bump into the cart and spook, or any number of stupid horse tricks.