Two levels of options in add to cart form
This is not how Drupal Commerce normally works w/ attributes - you typically define all of the possible variants as SKUs, associate them all together on the Add to Cart form, and show a form with both attribute select lists shown at once. You can alter that form to your heart's content, though, hiding one select list until another is adjusted - but note that Drupal Commerce also doesn't support an attribute not having a selection out of the box. This is a technical limitation of how the form is designed to work, but there is an open issue in the queue trying to support that.