Sometimes you don't even need the other session. You might only need one. Session A works directly with what you percieve to be the problem. That may be all that needs to be done. Session B Takes your underlying fears, limitations or what's missing in your relationship.
With session A you handle the conflict directly.
With session B you're looking at waht you are afraid of, what's missing and what there isn't enough of in your relationship.
You might decide, you experience conflict with her because you don't feel appreciated, as a limitation you might see it as you're not showing her your appreciation for her and you might just decide that you lack appreciation in your relationships.
Deiced what would be the ideal you want, what you are willing to give and what you would like to receive in return.
You may have everything you need with either session A or Session B. A does not mean it's the first session you must listen to if you know where you're feeling pained in a relationship. Listening to the session might trigger some insights taht lead you to the other session.
So if you have a reason for listening to session A but can't think of one for session B. Stick with session A.
But when you listen to session A and afterwards or still find yourself feeling helpless or hopeless about changing the situation. That's exactly the feelings you work with Session B on.
Alex