The various models of the Sit n' Stand are the only stroller specifically made for this purpose, though Peg Perego has a little ledge on some models that allow another child to stand for short periods (it's only about 3 inches wide.) Try getting a used S&S if you don't want to pay full price. (They are very heavy, lots of people find that they are too difficult to transport and get rid of them.)
You can also use attachments such as the buggy board on the back of a single stroller to achieve the same thing. This sort of attachment will run about $50 if you can find it on sale, $80 if you pay full price.