Think of it this way: the companion fare makes the price of a second ticket $99. That's all. ANYTHING else you want to do with a ticket is the same as if you had bought a normal 2-person itinerary.

When you buy a first class fare, you can change the departure or the return at any time. When you purchase a ticket, you can buy any available flight and change it later, but you have to use those funds within 1 year of purchase. You won't be able to use the companion fare for a flight greater than 1 year after the expiration of the companion code.