Southwest tickets can always be changed before departure with full credit for the money you paid. In the unlikely case that your flight is sold out, you can change your ticket to one that still have two seats available. It may be more expensive than your original one, but so be it. If your wife will be flying Southwest in the coming year (apart from the planned trip), you can also buy a ticket for her now, and then cancel it when you have your companion pass. She gets full credit for her cancelled flight, and the cancellation frees up a seat that can be used for the Companion Pass.