Forgot to mention, since we'll be using longer posts, that means dialogue between two characters should be in a single post. The best way to have dialogue between two characters owned by different people in a single post is Titanpad, a website where you can work on a single document simultaneously. to use it, got to the site and click the link in the center of the page to go to the page where you write, click "share this pad," copy the link and paste it to any people you want to post with, and the page will be available to edit by everyone you gave the link to. the site's link is down below. I'm learning how to use it myself, but feel free to ask questions about it. If you have any substitute methods of collaborative posting, let me know in PMs.

https://titanpad.com/