Two parties (Advice)
One technique I've seen used successfully, in lieu of groups, is to label threads with the names of the PCs allowed to post in each. You can establish a continuity of events by noting the Season, Episode, Act, and Scene numbers... e.g. S01 E02 A03 S04 would be Season 01, Episode 02, Act 03, Scene 04. Tack on a location and the PCs present and you then know who is where and when. Two threads with the same SEAS are unfolding at the same time, and you could further refine that with a lower-case letter after the scene number. That way, you can handle things like flashbacks without mucking up your timeline.