See text at page 16 for full fact pattern. Barbara is the one who must take a legal action, but if she is your client, you must see her and know this is what she really wants. You still would have to see Barbara, be sure that she agrees with Natalia, believe that there was no coercion, predict she will not change her mind, and get a waiver of the conflict. That is a lot of conditions to meet.
Joint representation seems unwise.
If Barbara and Natalia really agree, it is probably better for both that you represent Barbara alone. In Variation B, you are presented with the question of Barbara's competence. If you see Barbara in Variation A, you may find the same problem. If you are representing Barbara, this raises whether you believe she is still competent to speak for herself. However, if the client really is not competent, you would have to consider who could legally act for the person.