Can you use vegetable oil instead of canola in a party mix recipe using saltine crackers? 
Double-yolked eggs are the result of an anomaly in the egg generation process in the hen.  They can happen in any breed of hen, on any feed.  It is a result of two ova being generated at the same time, and then encapsulated in a single shell.  According to My Pet Chicken, it happens more often with younger hens.  I am not aware of any process to intentionally foster double yolked eggs, but that doesn't mean such a thing doesn't exist.  They are safe to eat, although they may be visually startling. 