Drupal commerce and multiple products sharing SKU
You can handle that with Commerce Product Bundle. Some details about it, from its project page:


  This module extends Drupal Commerce by allowing the site admin to
  collect a number of existing products together in a bundle, and offer
  them at a new bundle price, with a new bundle SKU.
  
  You can define products that consists of other products. They are all
  combined in one add to cart form which works like the core add to cart
  form. The bundle is then represented by one order line item. This
  allows the maximum compatibility with other modules such as shipping
  or payment modules.


As an alternative, there is also Commerce Bundle. Some details about it, from its project page:


  Implements product bundles for Drupal Commerce. For example, you want
  to offer products A, B, and C for a lower cost if purchased together
  rather than standalone. Notable features include setting the prices at
  the bundle group or product level, and product attribute support.


Note: if you wonder how those to modules compare to each other, then this might help (= quote from the Commerce Product Bundle project page):


  Commerce Bundle also allows creating product bundles. Customers add a bundle to their cart, but the component products show in checkout and order history. It therefore doesn't allow assigning an overall price to the bundle (but it has some neat ways to assign prices to component products). With Commerce Product Bundle, the bundle, not the component products, show in checkout and order history. Therefore an overall price can be assigned to the bundle