What do eggs with two yolks indicate?
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.