Does a companion ticket book into the normal frequent flyer fare class, or is it any available seat (since the is already a paid business or first ticket)?

I'm looking at trying to use some Amex points (converted to Asiamiles) Syd-CDG, and am happy to buy one J or F ticket if points can be used for the other. However dates aren't flexible so if a companion ticket books into normal frequent flyer ticket fare buckets, I don't like my chances.