Many times I see long discussions on the main site being conducted in real time, in the comments.
This is a rather non ideal way of communicating1 as many a time one is still typing a long comment and receives multiple replies while doing so, making it confusing.
Instead, create a room in chat and link to it in the comments. You can have much more free discussion in this environment. It doesn't get cluttered like the comments, and it's more amenable to back-and-forth conversation.
Besides that, it works for non-realtime discussions as well, and works for many users chatting just as well as 2.
(Also, discussing in chat means that there's no worrying about the comments getting deleted later on. Chat discussions are preserved)
Besides chat rooms for individual discussions, we do have a main chat room that is reasonably active. This is usually just a place to hang out and discuss physics, the site, or pretty much anything else.
1. besides not really being allowed; while short discussions are OK, long discussions shouldn't really happen in the comments.