it is possible to add 2 items at the same time using the ecart add to cart server behavior.
Create the form with the add to cart button on it, then in the server behaviors panel, click the plus button and select ecart -> Add to Cart -> Add to Cart
set the trigger to Button: <your button name> Pressed (where <your button name> is the name of your button).
then on the binding panel, set the product information for the first product.
after clicking ok to add the first add to cart behavior, add another one for the second product.