When scheduling talks in a conference session, what is the etiquette, if any, regarding placement of multiple talks from the same research group?
You have to plan the symposium from the audience’s perspective, not the individual research group’s.

Presenting multiple talks from the same group back-to-back or even within the same session can become tedious, even if the work is nominally “distinct.” If I were planning multiple sessions, I would avoid putting abstracts from the same group in the same session, but instead spread the pairs out evenly among the three sessions (if possible).

If your sessions are thematic, I’d still try to separate the talks within the session, so that the talks are not back-to-back, but perhaps “bookending” the session or nearly so.