Add the add to cart button for the first item, then add a checkbox to the add to cart form.
set the value of the checkbox to the ID of the second item to add.
Create a lookup recordset that will filter the products table on the ID passed by the checkbox when the add to cart button is pressed.
Then use an add to cart server behavior. On the server behaviors panel, click the plus button and select eCart -> Add to cart -> Add to Cart.
Set the trigger to if the lookup recordset is not empty
on the bindings panel, use the columns from the lookup recordset.