I usually go by feel. Some days it feels more natural to focus on one language or the other. So I adjust depending on that. It is more important to maintain the long-term view; that progress will be made over hundreds of days / conversations, and that any one conversation is not that important. Having said that, both people should show up with some goals and ways they would like to approach those goals.

IMPO, each one must propose what they are interested in learning, then the other person can teach around 10 phrases, verbs, nouns, or whatever you agreed on. Person A can explain it in the other person's (B's) preferred language or in their (B's) target language, depending on what she (B) prefers. Then the other person (B's) does the same. Working with around 10 new things to learn is what better works for me. My brain picks up on it and remembers it easily.