In situations like this or in dialogue-heavy scenes, I tend to be a fan of making collabs on Google Docs or so. After leaving it open for about a week, and letting people add smallish bits in turns (sentences or 1-2 paragraphs at most), it eventually results in a decently sized post that can be submitted as a chunk. That's how I'm doing it in my RP anyway; I believe I recall Jack saying he wasn't terribly fond of them? You can let me know what you think.