IT is the same processs for adding the second add to cart button as it was for adding the first.
Place the curisor on the page where the add to cart button should go.
go to Insert -> WebAssist -> eCart -> Add to Cart button.
sounds like you are using Static products, so select the Add Static option. set the Go To page to the cart page.
on the bindings panel, add the information to the cart columns for the product.