Can I chat with more than one companion at once?

Yes. Make a group from the Create tab and add the companions you want in it. The reply comes back as one message with each speaker’s name in front of their own line, so a scene can move between several of them in a single turn. Your plan sets how many groups you can have, and Usage shows where you are. A group is private to you, and deleting a group never deletes the companions in it.