Is it possible for customers who buy Item A and not also Item B to be reminded at checkout? I.e. You have not selected an Item B for your purchase. Would you like to continue shopping. If they click yes they would go to the Item B pages. We do have 'You may also like to add....' on the Item A page, but this could be ignored.
I think that it can be done, in the view "checkout / cart" check if the item b is present in the cart, if not present display a popup with the 2 buttons, one redirecting to the B product page, and the other one which just close the popup.
