Welcome to Flyertalk, sroloff!

You designate your Companion first (when you are awarded the status), then book her flight.

Your Companion can get the last seat on the plane. So, assuming you are buying WGA fares, buy them for yourself and non-Companion, then book Companion.

If you are worried about the plane selling out, book your future Companion NOW with points, then cancel that reservation after you can get the Companion ticket.

With Southwest, it's almost always best to buy all tickets one-way and one at a time. Makes it easier to do subsequent changes.