Not sure how your addProduct Page is setup, I'm assuming, the user goes to the desired products Page, which has multiple items listed on it --> adds two items and then clicks add to cart button? If so, in theory:
form action="addToCart.cgi"
prodKey1Quantity = 2;
prodKey2Quantity = 2;
submit (addToCart);
addToCart.cgi should run the commands to add to the DB and display the results/modification page.
HTH