Juste use the 'Auto Product Display' module (http://drupal.org/project/commerce_auto_product_display)

Just use the 'Auto Product Display' module which simply Product Display creation process by providing option the create it at the same time with Product creation.

Features

• Provide the Product Display creation form along with the Product creation form. User can fill that form to create Product Display at
  the same time with Product creation.
• That form, which used to create Product Display, will be the same as the form when user try to create Product Display manually. So, If you have additional fields in your Product Display node type, those fields will also appear in the form.
• If the product is referenced by only one Product Display, the Product Display content is displayed directly to the form, so it will be automatically updated when you save the product.
• If the product is referenced by more than one Product Display, the referencing Product Displays will be displayed in list, so you can
  also update the Product Display by clicking the link from that list
  first.