Let buyer purchase different product options in one order

I'd like for my buyers to see the option to put multiple product options into the eCommerce shopping cart and choose a different quantity for each.

For example, if I offer T-shirt (red) in sizes S - XL, one customer should be able to order two shirts in an S and two shirts in L. Right now, I would have to make each size a different product to make this possible. Please add this feature!