for the discount, you will need to add an additional column to the cart object to mark the items that the discount applies to. For the sake of example, lets name this new column "discountable"
In the add to cart button bindings, set this column to a value of 1 for the 2 items that the discount applies to and 0 for all other items.
Now when creating the discount rule, use the following settings:
Number Of items with a specific column value:
If the number of items where 'discountable' has a value of '1' is '>=' a value of '2'
Based on items With A specific value:
Total Of 'Quantity' Where 'discountable' has a value of '1' 'time' '0.1'
to add the 2 items at the same time, use the eCart Add to cart server behavior.
Add a form and a submit button to your page, then go to the server behaviors panel, click the plus button and select eCart -> Add to Cart.
Set the trigger to Button <Your button name here> pressed
on the bindings panel, set the product information for the first product. Repeat this and add a second add to cart behavior using the same trigger, on the bindings panel add the product info for the second product.